This article goes over picking the ROM you'll be using. Feel free to skip to the next article if you already have this set up.
Alrighty, first off, glad to have you here, it makes me happy to see other people take interest in ROM hacking :) Secondly, game companies are pretty uptight when it comes to ROMs being shared around. This webpage is pretty much guaranteed to never garner any attention from Nintendo, but to be safe, I won't be posting any direct download links, sorry. (A quick Google search should do you quite a bit of good, you didn't hear that from me though.)
Before you download, consider which ROM type to use. You've got 8 to pick from.
Encrypted ROMs are for playing on official hardware and decrypted ROMs are for emulators. I advise using encrypted ROMs. They function well on DeSmuME and work on official hardware. Plus, you can always use decryptors when playing on emulator. If you have no plans of playing on official hardware and distributing your hack, decrypted ROMs work well too.
Now, game version: Black, White, Black 2, or White 2. BW1 and BW2 are fundementally very different; ui, flags, scripts, locations, and much more change between the prequels and sequels. You should consider ui, avaliable forms*, battle facilities, and progression** when choosing whether to use BW or BW2. When it comes to picking which version you want to use, you're mainly gonna be focusing on story again. Version exclusive Pokemon, locations, and ui colors can be altered easily.
Once you've chosen and picked up your BW/BW2 ROM, unpack the .zip that it comes in and keep it in a backup folder. You always want to save plenty of backup ROMs incase anything goes awry. Make a copy of the downloaded ROM, this is going to be the first revision of your ROM hack. Feel free to name these files anything you'd like to help you remembers what they're for/what version they are.