As said a good* accountant will be advising you and you make the choices. Some just get you to give them everything with no advice, they make all the decisions and fiddle with figures and tell you nothing, you sign stuff without understanding and then if it’s wrong revenue hit your ass as you signed it off.
* – good ones tend to cost more 😉
Expenses you can claim – all kinds, but the classics are travel, subsistence, accommodation, purely business phone use, business supplies & equipment, use of home.
Absolutely must keep receipts. Accountants don’t need them, but revenue does. Keep for 6 years. If you claim mileage, keep a log of all mileage.