I want to build my own patio. How do I begin on the formwork? Can I use 2x4 or do I need 2x6

You can use 2x4's for the forms. What you want to do is dig down about 8" where the patio will go. Install 4" of 2B stone and compact the stone. Set your forms so the top of the form is 4" from the stone. Make sure the stakes (wood or metal) securing your forms and pounded in the ground, are flush with the top of the form. You'll need them flush so when you screed the concrete the screed board doesn't hit any stakes. This will make the concrete even with the top of the forms. Make sure the forms have a pitch. You don't want a level slab or water will just sit on the slab and not drain off.
Before you start the pour install 4x4 weld wire mesh. This will help reinforce the pour.
After the concrete starts to set, trowel the surface smooth with a bull float or a magnesium trowel. This will get you a nice smooth finish.
The next day you can't to cut control joints in the slab. These can be evenly spaced. For a 20' pad usually you should have 2 joints cut in.