IOS Macbook Air clean up and new set up

My Macbook Air got very slow, operating software is too old but new one would occupy too much memory. I-cloud doesn't synchronize and occupies lots of memory as well. The entire PC is probably full of junk. I need somebody to help me clean up the computer, eliminate unnecessary junk and probable set it up new from scratch. Also Icloud should get updated so that it synchronizes with my iphone. It should be possible to do all this via teamviewer. Operating language is german.