Just like you would on a laptop or PC. If you don't have an account already with the online poker room you want to play on, you'll first have to create one. Then you'll simply need to enter the appropriate information such as your credit card number and security code to make a deposit of the amount of your choosing.
Once that information is entered, withdrawing is super easy as well and usually requires no more than a couple of clicks on your Android. Note that if you have an account with an online poker site (that has money in it) and then download their app, your money will be right there on your phone when you start playing.