Earstretching.net

First of all, guns really aren't suitable for piercings; getting your lobes pierced with one may have created scar tissue that will make it hard to stretch.
That being said, most "standard" earrings are 20-18G, meaning you should start at about 16G. Some people who've had lobe piercings for a while find they've stretched over time and can fit bigger jewelry in. If you had no problem fitting the 12G in one lobe that's fine, but don't force it into your other lobe. You may need to start at 16G or even 18G on that side.
As for your whole stretching journey, here's my advice: Wait 6 weeks or more between every size, massage your lobes with an oil like emu, vitamin E, jojoba, etc., don't skip any sizes, and don't wear tapers. If you stretch slowly enough you shouldn't be creating wounds that you need to "clean," but you can clean your jewelry with antibacterial soap and water.

I had my lobes pierced with guns (when I was too young to know better) and I've never had any problems with them, but I agree that you're much better off getting pierced with a needle. I'd never get anything else gunned.

Mine were about 1.2mm (not sure what gauge that is), but stretched up to 1.6mm pretty easily 'cause I'd had them done so long.
I agree that you definitely shouldn't force it, try a smaller size and start from there.

corazon_de_oro has given really great advice, basically. :b

Thanks for the reply. I wish I would've seen them before I put the 12 all the way in. It's healing great though. Doesn't hurt anymore, no more redness. And I do have jojoba oil for it. And I've had them both in for like three weeks now. Would it be alright if i went up a size?

I'd leave it longer. For a normal stretch I'd say wait about 6 weeks between sizes, but as you've ripped your ear I'd recommend even longer.