Flat rate is where you pay a smaller amount of VAT out to HMCE so for instance 12.5% for management companies (it is different for all businesses) and you don't have to account for VAT on purchases then, less work.
If you have bought a decent amount of equipment over the last 3 years you are probably better of doing it the normal way for a few months and see where you are, then transferring to the flat rate later on, maybe next year.