When applying for a mortgage, it's a great idea to get ahead of the game and try to sort out the things that you are likely to need, especially now that things like bank statements and bills are often only available online.
While what you will need will depend on your personal/current circumstances and the lender in question, this brief checklist should give you an idea of where to start.
1) Check your credit score and report.
2) Make sure that you can evidence where your deposit is coming from and a 'build up' of funds for at least 3 months
3) Ensure that you have the relevant documents to prove your income (for employed, 3 months, for self employed, minimum 1 year)
4) Double check that your ID and address is up to date on all accounts and bills.