Cake pans are usually measured across the bottom of the pan, even though some are slightly flared at the top. If the recipe calls for an 8" pan, use a pan that measures 8" across the bottom of the pan. If you use a different size it will affect the baking time, and the cake won't be as tall as it should be, obviously.
To make sure that the cake comes out of the pan, line the bottom with waxed paper, parchment paper for baking, or a silicone pan liner. After cooling the cake in the pan for at least 10 mintues, run a knife around the sides of the pan, then turn the cake out. When you turn the pan over, the liner will release the cake and it will come out without any problem.
You don't need to buy super-expensive pans, the regular aluminum ones conduct the heat evenly, cool off quickly, and will give good results. I've never seen a bakery that used the fancy "professional" pans you get in department stores.
Use the top of the pan to level off the cake before you remove it. Take a long bread knife and cut the domed top of the cake off, using the top edge of the pan as the guide. It will give you a level surface to work with when putting the cake layers together. Just be careful not to scrape the pan itself with the knife, or you might add some metal shavings to the cake, which is generally not something you want to do.
When using square pans, they rarely give you perfectly straight edges, so you can trim the sides after the cake has been filled and before crumb-coating. You can also trim the edges off of round cakes to make them easier to ice if there are a lot of excess crumbs on the sides.
When you are moving pans of batter around, make sure to keep a tight grip on them, or they might randomly flip out of your hand and slop batter all over the floor. Not that this happened to me last week, or anything...