Just pick something you’re going to enjoy collecting. The vast majority of Pokémon cards are available for under $0.25, so you really don’t need any sort of fortune to get into Pokémon. It’s only when you come to the high-end cards where things start getting a bit silly - but that can be said for pretty much any hobby.
Knowing where to buy will also save you a lot of money. @quuador 's post here may be a good starting point: www.elitefourum.com/t/where-do-you-buy/29408/20.