Ah, yeah. I didn't really make the connection between this post and the post where you were asking for advice on starter equipment. Basically your airlock will have a small lid on the top that you can pop off and pour liquid into. If you can find a close up picture of an airlock somewhere, you'll see what I'm talking about.
As for using water+sanitizer Vs. using vodka, it's a personal preference thing. I like to use vodka, simply because if I run out of vodka, it's not the end of the world. If I use up all my sanitizer filling airlocks, come brew day I might be screwed. Vodka does tend to cost more (since you'll only need a few drops of sanitizer into an airlock full of water) and it does evaporate faster so you'll have to keep an eye on that and keep it filled.
Oh, almost forgot, your airlock should have a line on it that shows you exactly how much liquid needs to go inside. Overfilling isn't a huge problem, although a few drops might bubble out.