you will need your company UTR (unique tax reference) This is a 10 digit code that will have been issued to the registered office address of the limited company.
Self Assessment - This is the tax that you owe as an individual that hasn't already been through PAYE. You need to pay your self assessment bill by 31st January.
To pay your self assessment bill you will need your 10 digit UTR number. If you are using online banking you will need to enter K after your UTR e.g. 0123456789K