First, we would recommend that you pull your credit record to see what it looks like and whether it is in good standing or not. It is also a good idea to check your credit record regularly to make sure everything is as it should be.
If you see that there are accounts whose status are in arrears you will have to pay the arrears, and then make sure that you keep up to date with those accounts going forward.
If you have Judgements or Administration Orders on your profile you will need to settle the outstanding debts in order to have that information removed.
Avoid applying at various creditors for credit in a short period of time, this will affect your score negatively. Furthermore, every time you apply for credit and it gets disapproved it also negatively impacts your credit record.
The best way to improve and keep your credit record in good standing is by paying the full instalment that is due on time every month and checking your credit record on a regular basis.