It took me a while to get to grips with it all!
Simple sources are proof of the person. Birth, marriage or death records and census records are the basic ones. But pictures, newspapers, books can also be sources!
For your basic sources I find the easiest way is when you are on the profile you want to source click on the drop down menu (top right) with their wiki-id. one of the options will be research. Click this and you'll have to sign into rootsearch and in turn this will sign you into familysearch when you select it. This is all free!
Familysearch will bring up a list of possible sources for your profile; when you find the right one scroll right to the bottom of the source and there is a down arrow that will show you a source citation, simply copy and paste this to your profile and you can access the source whenever you want! All you have to do is click on it!
It sounds a bit complicated but it does become second nature, happy to help if you need me to in anyway!
Checkout some of the tutorials on help pages to they helped me massively!