There are numerous Open Source licenses, but the ones I'd recommend are either BSD-style or the GPL. You'll have to decide which you like. Should people be able to take what you've done and wrap them into proprietary software and sell it, without necessarily giving back their changes? Up to you. A BSD-type license might get you more users, and a GPL-type license might get you more development help.
If you're thinking of dual-licensing, with an open source and a commercial license, you almost certainly want to go GPL for the open source license, since BSD-style doesn't leave you with enough extra rights to sell.
You keep the copyright on everything you do that's not for hire, unless you explicitly give it away. If you start getting help from other people, you need to decide what to do. You can ask for the copyright to be transferred to you, which will keep your complete copyright control at the expense of discouraging outside developers. You can trademark your software, to keep the branding, and this is independent of who owns what copyright. I'd advise talking to a lawyer about that, as trademark law isn't as clean as copyright law, and can vary from state to state. If you live in the US, you can probably get a lawyer referral from your local bar association, and it shouldn't cost much for an initial consultation.
As Jonathan said, check the OSI for information on possible licenses. Pick one from there. They'll all work, more or less, unlike a license you might write up (unless you know what you're doing). Some sites, like Sourceforge, don't allow projects that are not under an OSI-approved license, so you'll get more options with an OSI license. Moreover, lots of people are already familiar with the standard licenses, and you won't have to explain your license to them.