Depending upon the number of drums you have will dictate the number of channels you need. But here are some things to consider.
You can go a LONG way for little money if you buy smart. I currently have several mixers, and have gone through some, too, including Yamaha, Alesis, and Mackie. I was recently given a Mackie CR1604VLZ. This is a great way to go to start out, and them some - if you're on a budget, even better. 16 channels is enough for most drum setups, if not too many channels. But, consider that once you go down the road of hearing your drums mic'd through headphones/earbuds/IEMs, you will NEVER want to go back.
The 1604 is dirt cheap nowadays, used. They go for about US$200-$300, and are built like a tank. Not only does it have 16 channels, it includes inputs for your music-listening device without compromising a channel, so all 16 channels remain. I recently put this same mixer into play in a 5 piece band - guitar, bass, drums, keys, and dedicated lead singer. My drums take up 9 channels, 4 of us sing, and bass, guitar, and keys round out the 16 channels. I still have an .MP3 player in the dedicated input. It's got headphones out and you can choose from several different things to listen to quite easily. You can listen to your drums and your .mp3s at the same time, too, which is what you want right now. This mixer can also be rack mounted vertically in a standard rack, which is quite convenient, as standard racks are cheap. You can even make one yourself.
When you're ready to transition to PA use, you will not have to buy another mixer. Get an amp and speakers and you're set. Sure, you can spend a lot more for a "better" mixer, but from my experience with this mixer, you can be more than happy with it for quite some time, especially if you get a used one in good shape. WAY better than even some others you can buy new. Not only did Mackie think about flexibilty when designing this line of mixers, Mackie also does a good job with their user guides, better than any other company does, that's for sure, so if you're just getting started you will learn a lot from the Mackie experience. You can grow from there if you want, but you can also do plenty with that mixer.