Hi,
As above is quite important as there's a few things that can trip up people that have raced other classes and it is best to get it right first time. There's some good, helpful people at the Milton Keynes club so go there and have a chat with them. I'll try to explain some though.
As far as the Atom is concerned, initially build it as kit, the base setup is good, as are the supplied tyres. Make sure you get the Pro kit though.
Only 13.5 motors and 1S cells are allowed. Most clubs run to the BRCA lists. The Core equipment is on the list and is good to start with but you'l probably want to upgrade it at some point as you progress. Life's easier if you get a 1S compatible ESC so you don't need a booster.
BRCA approved cells motors and ESCs are at
https://www.brca.org/sections/eb/.
You want the brushless motors 13.5, 1S stick LiPo and approved Blinky ESCs lists.
Tyres are foam. It is possible to run kit sizes but it is better to true them down. The new Atom tyres are available at trued down sizes so makes life easier again. Otherwise there's always someone willing to true a few for you until you have your own equipment.
Additive, almost all clubs run to the BRCA list here
https://www.brca.org/33-approved-add...-2015-2016-pdf. Other additives can damage both tyres and carpet. It is best to learn additive use from someone else but it is not too hard when you get used to it.
That's a start off the top of my head.
Happy racing!