Hobgoblin

Hi. Im wanting to buy a 8'x6' metal shed. It wont come with a base, so im thinking it best to lay a slab in my back garden to sit the shed on. I will dig 8'x6'x3'' deep plot and level it all out, then create a woodern frame, and poor concrete so that i have 3'' below and 3'' above ground level ending up with a 6'' deep slab. (PLEASE IF U HAVE ANY TIPS, SUGGESTIONS, BETTER IDEAS PLEASE SHARE THEM) just wondering if i used a 1:2:4 cement, sand, gravel mix (PLEASE IF U HAVE ANY TIPS, SUGGESTIONS, BETTER IDEAS PLEASE SHARE THEM), How much of each, cement, sand, gravel i would need to do that job and where and what shud i buy? im not a builder, i do understand there are types of sand and gravel and wouldnt know what to buy, can you help? Any suggestions on my plan please share, ive only planned this after perhaps 2 hours comprehension from other ideas on how to erect such a shed. Thanks, Hobgoblin