when you say "self employed" I assume your going from PAYE to what they used to call a CIS4, now you don't get a CIS card you get a UTR number.
you can basically claim for anything that you use in the course of your business. that's clothes, tools, you can claim a part of your home as an office (i claim office costs of £250 pa). you can claim all your accountancy fees, you can claim for digs if your working away and up to a £5 a day meal allowance. you can claim for any computer items printers you use for business, any office furniture etc. you can claim your mobile bills, part of your braodband and telephone if its also used for home use etc. by that I mean if you use the internet to look for jobs, send invoices etc, you could claim 50% of the total cost of your braodband back.
Cars
you can claim transport in one of two ways. You can claim a capital allowance for your vehcile, say £5000, then you claim tax relief against that. then you claim for your insurance tax etc,
OR
Mileage
you claim 40p per mile for the first 10 000 miles, then claim 25p a mile after that. that covers mainteqnce, tax, petrol etc. it's better to do it this way if your car isn't worth a lot.
you can also claim bus/tube fares etc
you must keep your records for six years and if you do get investigated (highly unlikely but it could happen) and you can't account for the claims you've made, you will have to pay back any rebate they have given you.