Don't forget about good ol' push-ups! Even if you can't do regular push-ups now, a great way to ease into them is to perform them on a slightly vertical surface (like a bench), and as you get stronger move to more and more horizontal surfaces...til your bangin' out those bad boys right on the ground. Yeah, your friends will be impressed
Just think of push-ups as "moving planks".
Also, I see women doing push-ups incorrectly a lot. Make sure when you are doing them that your elbows come back at a 45 degree angle, don't let them flare out. It's a bit harder at first, but it puts A LOT less pressure on your shoulders and will make sure you don't hurt yourself in the long run.