I've gone into bars by myself. I've even gone out to clubs by myself. When people are drinking, they get very nice & friendly. It's easy to meet people. Don't bring a book, that's only going to isolate you. Sit at the bar, have a drink. Then come up to someone - anyone, or even a group of people, and strike up a conversation. The conversation starts with, "Hi, I'm Angie." And then you can say anything at all, the truth works well. "I just turned 21, it's my first time in bar." This will get a response, that's probably all you need. A few times when I was at a bar that had a semi-private party - everyone invited there to celebrate someone's birthday, I might say something like "My friends invited me here, and I can't find them. I think they're standing me up. That's very awkward. Can I hang out with your group for the evening?" Then, once you know one person in a room, it's perfectly appropriate to ask them to introduce you to others. Just keep in mind that people go out to bars to be friendly and socialize, so don't worry about imposing yourself on anyone - that's the very point. If they didn't want to be bothered, they would have stayed home.

As to gay bars, I'd look online for clues. Also, you can go to any bar (one populated by young-ish people as opposed by old geezers), and then ask. You can ask the bartender, or any strange person who appeals to you whether they know of any gay bars in the area. You are sure to get a response.