The average size of a facebook cover is about 851px × 315px , if you're good at photoshop , you can download a few brushes , new fonts and work your way around it using your skills to make something cool easily.
You can also use pre-designed Facebook covers and if you want it to design yourself, then you can search it on Youtube or Google.
Create a document 850 x 320 px on Photoshop. Then continue art by using your imagination, use attractive fonts.
Just search youtube http://www.youtube.com/results?sear...3.0.107.790.10j1.11.0...0.0...1ac.76JWiTJtN_s I did it for you
Another simple way is to print screen somebody else's timeline picture and then change it with yours.
You can use 915x315 as image size! because will fix on cover of Facebook page. It's the actual size what Facebook support.