Your best bet would be to undertake small, regular credit commitments like TV License, Utility Bills via Direct Debit payment. Make sure you keep up with these though..........just as timely payments improve your status, late or incomplete ones will have the opposite effect!
Secondly, ensure that you never slip into your Bank's overdraft facility as this will penalise you further. Try and set some aside every month to put into a Savings Account, even the smallest amount will show your bank that you are managing your money responsibly and, given time, will improve your scores.
All you can really do is manage your money effectively, if you don't owe money then it's unlikely that you will be considered for credit as there is no history of you repaying it and this is what lenders are looking for............owe too much though and you'll find further credit impossible.
It does take time to re-build or repair Credit Histories but when done properly it can be painless enough and the rewards are self explanatory.
Good luck!