The first thing you need to do is buy a good book. Narrow down your choices, and go to your local library, or if you cannot get a book, get online (but be absolutly sure the website you are looking at is sound and reliable, because there are so many that are not)
I reccomend a ball python. They are docile, thick bodied, and easily handled- if you get a CBB (captive born/bread) from a reliable breeder. Go to a herp show if you'd like to see the difference in the various snakes. But snakes are a commitment- husbandry is important- proper humidity, temp, cage set up, etc. Feeding in a seperate cage so the snake does not associate your hand coming into his/her house with food is important, also. So research is a must!!!
I've been keeping snakes for over 10 years and I love it. But remember, be careful, no matter what you get remember this, anything with a mouth can bite!!! (Although my ball never tried)
ETA: I can't stress enough... RESEARCH, RESEARCH, RESEARCH!!! Oh, and once you've made your choice, HAVE FUN, it's totally addicting!!!