What size is the enclosure? What do you like in a snake... Bigger, smaller; laid back, active? Each species has its own pros and cons. There's several different options that make great beginner snakes, depending on what YOU want in a snake. My recommendation would be to choose a few species you're interested in. Research them, and narrow them down by what ones are of a care level/size/etc that you can handle. Once you have your final list, go to a pet store, an expo, or find a keeper with those species, and see if you can handle a few of each--especially adults, to get a feel for what you can expect.
As for good beginner species, there's a few great ones to look at. You brought up ball pythons and boas, so I'll touch on them first. Both are great snakes.
Ball pythons can be a little difficult with their generally picky feeding habits, but are otherwise great beginners. They can be very lazy, not often active snakes, which may be good or bad for you. Personally, I'm not a fan, but I know plenty of people who love that about balls. For a ball python, you'd be looking at an approx 3' enclosure for the average adult. A good care forum:
http://ball-pythons.net
Boas, as has been mentioned, can get quite large, so housing them can be tough... For a male Colombian Boa Constrictor, you're looking at a 4' enclosure minimum. However, putting aside the size issue for now, they can make great beginner snakes. Very tame, straight-forward care, total pigs for feeding. One thing to note, if you want a boa but don't want the size, is there are dwarf localities that stay close to the size of a ball python. Effectively eliminating the size concerns. Hogg Islands and Nicaraguans are two. Nics stay in the 3-4 ft range on average, while female Hogg Islands can still get over 5' but rarely over 6'. Even for the dwarves, a 4' enclosure would be ideal.
Then there's the other "beginner" staple of the snake world, which has been mentioned a few times already: Corn snakes. These guys stay small, around 3-4 ft average with specimens up to 6' rare but possible. They are a slender snake, never needing prey larger than an adult mouse. Very easy-going and tame, but they are active, never really sitting still for handling time. Their care is very easy, as they don't need any special heating beyond one good hot spot (80-85F), don't need any special humidity, and don't need a great deal of space. A 3' enclosure is ideal, but one can thrive in a 20 gallon Long (30x12x12") its entire life. King snakes were also mentioned. For most of them, their care is just about identical to corn snake care. They are also great beginner snakes. A good care forum for corn snakes (I'm sure they'd be happy to answer questions about other colubrids like kings, too):
http://cornsnakes.com
Best of luck with your decision.
