Yes it does sound like a joke ! if you are going to venture off into self employed bookkeeping, you of all people should know how to do your own books before let loose on clients !! Sorry to sound slightly bemused by your post but Im sure others may be thinking the same.
I suggest that you think about what records you will need to keep as proof of earnings / expenses, ie sales invoices when you invoice clients for your work, purchases ie, software, stationery, marketing materials etc., all these items make up the details you will need to submit your own Self Assessment.
Setting up the nominals in Sage should then reflect the types of expenses / sales you will generate as a self employed Bookkeeper.
Do you mean you are going self employed in a different industry and need advice on how to do your own bookkeeping? If that's the case I would suggest using a qualified bookkeeper to help you and show you the ropes. If you are looking to become a self employed bookkeeper then you really need to do some studying. Good luck
There is a lot of info dotted around on the site about using various versions of Sage etc. Have a trawl back through recent posts and you'll soon find loads of advice on registering, training/self-tuition on theory and use of software, exam tips, setting up in business, etc etc.
Your very first post says you are learning Sage through Pitman Training. It might be an idea to ask your tutor with them, as you seem to be struggling with the basics of Sage.
