O.K.  You didn't ask for it, but I'm going to give it to you anyway (wow, sounds like prom night).   This is a quick guide for selecting focal lengths for 35mm and full-frame digital cameras.  If your digital sensor is 18x24mm (standard format), multiply the focal lengths by 2/3 to get the equivalent focal length for the angle of view shown.  Some of the equivalents can be found here. General advice on lenses follows the focal length ranges.

Wide

I hate to break it to you, but in this range price is generally directly proportional to quality.  So if you want good lenses, get out your wallet.  If you want fast, good lenses, get a second mortgage.  Note also that lenses with faster apertures do very little to reduce depth of field.

Less than 20mm (diagonal field of view > 94º).  People really love to own lenses in lengths running from 6-19mm.  The problem is that is is nearly impossible to produce anything of value with an angle of view wider than 94 degrees.  This territory covers fisheye views of skateboarding, "intellectual" environmental portraits, and "interesting angle" photojournalism.  For most people, lenses in this range are useless.

20-21mm (94º).  This is still pushing it.  20mm is wide enough to take in gigantic objects from a reasonable distance without converging parallel lines, the sole catch being that you need to have the camera dead level - something you can do either with the framelines in a  rangefinder (generally), a grid screen in an SLR, or an external bubble level.  DO NOT buy a cheapo 20mm SLR lens for use in landscapes - you will be sorely disappointed.  Go for the most expensive one you can afford.

24mm (84º).  In the ever-widening modern world, this is considered a wide-angle lens.  Lenses in this length are some of the best wideangles made.  24mm gives you some breadth in shots, but be aware that you need to keep the camera aligned with the horizon.  These are relatively inexpensive for SLRs and a good value used.  For rangefinders, you can pick two any two of the following characteristics (a) cheap (b) fast, or (c) sharp.

28mm (75º).  The 28mm is currently thought of as the beginning of wide-angle.  A 28mm lens will be able to handle most subjects you will see on a trip.  In trying to capture larger objects, watch out for tilting the camera up, something that causes converging parallels.  From about 1.5m, the 28mm is acceptable for taking pictures of humans.  Closer than that, distortion makes for some impressive ugliness.  The 28mm f/1.4 Nikkor is an exception, being about 30mm in real life.  There are a lot of bad 28mm SLR lenses out there.  Most 28mm rangefinder lenses are OK, because they are generally really expensive (Leica) or really slow (Soviet Orion).

Normal

There is no correlation between quality and price for normal lenses.  There is a direct relationship between cost and maximum aperture for any era and any manufacturer's product line.  Jumping from f/2.0 to f/1.4 doubles the price, likewise from 1.4 to 1.2.  Canon has always been the speed king with its 1960s f/0.95 rangefinder dream [nightmare] lens, but the modern king is the Leica Noctilux 50mm f/1.0.

35mm (63º).  The 35mm lens is the new "normal" lens, unseating the 50mm.  If you could have one lens, this would probably be it.  Lenses with f/2.8 maximum apertures are generally good, cheap and easy to find.  Manual focus 35mm f/2 SLR lenses are thin on the ground.  35mm f/2 rangefinder lenses are generally expensive, and there are no bad ones.  In the f/1.4 speed class (if f/2 doesn't have you covered), these lenses become exponentially more expensive, whether as SLR lenses or RF lenses.

40-45mm (56º).  These are supposed to be "ideal" lenses, because the diagonal of a 35mm frame is 43mm.  They are good for general purpose photography and have many of the characteristics of 45-50mm lenses.  They also often come in compact "pancake" configurations that make them about half an inch thick.

50-57mm (51º).  In the olden days, every camera was sold with a 50mm lens (with the focal length actually being 52mm in most cases).  A 50mm f/2, 1.8 or 1.7 lens is the cheapest thing you can get, and in terms of bang for the buck, an all-round winner (at as little as $60 new in some mounts).  Everyone makes good 50mm lenses.  They had better - both basic 50mm lens designs date back 100 years.  The problem is that like Tri-X, a 50mm lens is the second best for any particular purpose.  It is, though, good discipline.  55 and 57mm lenses are generally found with f/1.4 or f/1.2 maximum apertures.  I suspect that the longer lengths help reduce vignetting and help the big rear lens element from hitting the mirror.

Telephoto

A few key pointers.  Telephoto lenses are good performers.  In general, the longer the focal length, the higher the performance gets.  If you are a portrait shooter shooting headshots, bear in mind that the length of these lenses only determines working distance.  As you get past 105mm, the minimum focusing distance of longer lenses reduces the effective magnification.  So the magnification factor of your 105mm lens at 1m is almost exactly the same as your 180mm lens at 1.5m.  For distant subjects, though, magnification goes up directly proportionally to the focal length.  Telephotos are surprisingly good for landscapes with 35mm.

85-90mm (28º).  An 85-90mm lens is considered a short telephoto.  This is a great length to do a head-and-shoulders shot.  Pretty much all lenses  (with the exception of some examples of Soviet rangefinder lenses) in this length are good; some are exceptional.  In a manual focus SLR lens, choose a 6-element f/2 or f/1.8 design.  With few exceptions, these are very well corrected and sharp at middle apertures.  In rangefinder lenses, modern 90mm f/2.8 lenses are exceptional and tiny.

105mm (23º).  Nikon made this portrait focal length with the 10.5cm f/2.5 P.C. Nikkor, a rangefinder lense with a Sonnar formula.  It subsequently went on to dominate this focal length with the successive Planar formulas (two versions of the f/2.5 and f/1.8).  The current 105mm f/2D AF-DC lens is the best of them all.  Other manufacturers made o.k. attempts at 100mm lenses, but Nikon pretty much owns this one.

135mm (18º).  This general type of mid-range telephoto was the standard for portraits from the 1930s to around the late 1980s.  This length of lens is good at compressing perspective, meaning making people's noses smaller.  135mm lenses of every stripe are cheap (at f/3.5-f/4.5 maximum aperture), plentiful and in most cases excellent performers.  They are also reasonably small and light.  You can also get f/2.8 and f/2.0 lenses, but they are huge and expensive.

180mm (13º).  This is really pushing it.  180mm lenses are big, have big minimim focusing distances, and are generally expensive if they are really good.  That said, just about every 180mm prime lens with an aperture of f/2.8 or smaller is an excellent performer.

Greater than 200mm (<12º).  If you are a sports photographer, bird watcher, moose chaser or gazelle predator, this is a great range of focal lengths.  For everyone else, these lenses are too big, too heavy and too slow to use.  If you must have something in the 200-210 range, I would suggest a cheap zoom lens just to get it out of your system.

Other issues

Can I do it with a zoom?  In one word, no.  Although affordable zoom lenses cross the focal length ranges shown here, there is no comparison between most consumer-grade zoom lenses and primes in terms of sharpness.  Unless you are willing to spend major money on a zoom, resign yourself to the fact that your $400 zoom lens set to 50mm will be trounced by a $19 Yashica 50mm SLR lens that is a two stops faster.  Ouch.

How much speed do I need?  Good question.  Under most circumstances with 400-speed film, an f/3.5 or f/2.8 lens will be fast enough. 

If you use a lot of 100-speed film, or use a digital camera with a noisy ISO 400 setting, consider an f/2.8 or f/2.0 lens, especially if you shy away from using a flash.

If you shoot mostly indoors, buy an f/1.4 lens in a 50mm length.  It will be a relatively cheap way to extend your capabilities.  Remember that in low light, camera shake from  a low shutter speed is much more of an is issue than limited depth of field.

How much should I spend?  A good rule of thumb is that your lenses should generally cost less than a car payment.  If there is a lens you just can't live without, it might equal a house payment....  Of course, if you are a pro and can write it off - hey, I didn't write this for you anyway!