Since you're looking at a 3 series, you'll notice that the back has a 3 digit number that starts with a 3. The 3 lets you know that's a 3 series. Take the 318. The 3 says it's a 3 series, while the 18 lets you know how big the engine is (generally). The 318 is a 3 series with a 1.8L engine. The bigger the number, the bigger the engine (more expensive and more prestigious). So, you'll know a 325 will cost more than a 318 if they're the same year, mileage, etc.
Since you're in the US, it's kinda simple. 318 is the 4 cyl. Pretty much all the other numbers are 6 cylinder cars. BMW is famous for it's 6 cylinder and that's prolly the one you should get. You'll see 323, 325, 328, and 330.
As for the letters after the 3 digit number, if you see "ci", than that's a coupe. "cic" is a convertible. "ti" is the little hatchback. Everything else should be the sedan.
I think there's a faq or something around here.