I'm thinking of making my very own 'hot air ballon' in my garden will i need any safety certificates ?

Question

also what gas should i buy

Answer 1

You use a propane gas heater to warm the air in the balloon.  Theoretically your could burn anything in the basket below the balloon to heat the air in the ballon. There is a certification process.  There FAA rules that apply

Answer 2

If you intend to fly it, certification standards apply.

If it's just for tethered display, it's a local matter.