Well... you don't
have to buy it, but you should because it's the legal thing to do and it gives money to the wonderful people who designed the program. I warn you though, photoshop isn't something you can just open and know like microsoft paint. If you're serious about it, and don't know where you can get some hands on schooling, I'd look for an online tutorial series on youtube. Just do the stupid stuff they ask you to do over and over, and before you know it, you'll have a pretty good idea of how it works. Here's a
link to one of the newer versions of the program, but I'd suggest you try to get an older version like Photoshop CS3, because the price is pretty steep. Hope this helps. :]
*edit* I'm pretty sure Adobe makes software for Macs too. :]