You either need to ctrl- , cmd - on a mac. This can also be changes near the bottom left of your window, there is a drop down that gives you options like fit, fit to 100 percent etc. I choose fit up to 100 percent.
FYI- In the main comp window you can also change the resolution of the comp, you can choose full, half, third, quarter, auto and custom. This has caught me out before!! There may be settings you can gets in after effects preferences too, but in this case I'm not sure you need it.
If you are new to AE I suggest you have a look at Adobes tutorials. There's a lot of great learning resources out there.
Hope this helps
You're looking at a layer -- not the comp -- and you're Zoomed into 400%. Change it to 100% and you're good... for that layer.
I get the feeling you're trying to use AE intuitively by fooling around with it, which is a very, VERY bad idea. With AE, you have to crawl before you walk, and walk before you run. So save yourself hours of frustration and spend a good deal of quality time here: