If you also want to try to keep a wild snake there are many things you must consider. A snake you know you can keep, are willing to care for, and would like to keep temporarily as a pet, then the consideration start. Do note that I said temporarily: please only keep a wild-caught snake for a short time while you learn about it. As soon as the fascination with it starts to wane please release it back into its NATURAL habitat.
If you are new to pet snakes, find out what you should consider before deciding on a pet snake, and what species are the best snakes for beginners.
When choosing a snake, you are making a long term commitment - many can be expected to live longer than 20 years. You must be willing to feed prey animals to your snake (though frozen, pre-killed prey is the best choice), and you will probably have to devote some freezer space to frozen prey items.
Snakes are very adept escape artists, so make sure you have an escape-proof tank, keeping in mind that snakes are persistent about finding and squeezing through any small gaps.
Get a captive bred snake from a reputable breeder, if at all possible. Wild caught snakes tend to be more stressed and prone to parasites and disease, and more difficult to tame. For more about the advantages of captive bred reptiles, see Should I Get a Wild Caught or Captive Bred Reptile?
You will also want to do a cursory exam of your snake to check for any signs of illness: see Choosing a Healthy Reptilefor areas and signs to look at. It also doesn't hurt to ask for a feeding demonstration, to make sure your new snake is readily taking pre-killed prey and feeding well. Ball pythons are somewhat notorious for having feeding problems, so this is especially a good idea with ball pythons (though if you get a captive breed ball python it seems less likely that feeding problems will crop up).