Registering self employed

I am already self employed and my wife also want to register herself self employed (she is writing books) She has already purchased some of her tools and kept receipts but was wondering if she can still claim for them even though she hasn't registered yet because she is currently finding out how to. Can she still claim for them even though they were purchased before her official start date ? Also, as she hasn't made any money as yet because it takes time, can she still claim expenses ?

Which is the best way she registers online ?

Thanks.