Looking at your picture, my first bit of advice would be decide where your path/s will go and where your new shed & bins will go and mark those out. It looks as though you have at least 2 well defined sides, think about getting at least a foot clear at the sides to stop encroaching weeds, consider if you will have beds down the sides for fruit maybe - consider sun/shade and try not to increase your shade unless you want to. The 4 beds started look a sensible size - try and keep them narrow enough to reach the middle without walking on the soil - normally about 4ft. and I would suggest tidying them up, composting and then covering with weed control fabric of some sort to save yourself having to re-weed before you sow or plant out.
Good luck, have fun and don't overdo it!