Ok. You need to do following:
1. First get registered with ACCA for CAT qualification. Pay the registration fee and get all registration documents and ID card.
www.accaglobal.com
2. Contact the college you want to study in UK OR colleges who teach CAT qualification. Try to find their official representative in Nigeria as local official agent will be good but should be official.
www.educationuk.com/www.britishcouncil.org.uk
You can also visit your local British council / education UK office to get more information. When you found out your course and college must check it with UKBA Tier 4 Sponsor register before paying any fee/deposit.
3. Pay the college fee/deposit to get CAS, which is a main document to get student visa. If you use local official representative process will be easier for you. You will get all documents easily and will get you fee/deposit refund quickly in case of visa rejection.
4. Meanwhile of the above process, get your passport and all educational document ready. Collect all the original reference letters/transcripts/certificate from relevant authorities and get them translated in English, if it’s in your local language. Arrange funds as per your requirement, roughly £7200+Course fee. Funds must be in your or your parents named account for last 28 days. Also get the correct photographs of yours as per requirement of UKBA.i.e on mat-finish photographic paper with grey/white background. A medical certificate of good health and TB certificate is also required.
Don't forget to check UKBA website to check what is latest student visa application requirement for Nigerian nationals. Keep all documents ready.
5. Download and print student visa form (VAF-9) and PBS Appendix-8 for general student from UKBA website or get it from college local representative or British council office. Please note its TWO separate documents one is VAF9 and another PBS Appendix8 general student which needs to be filled and attached with VAF9.
Read it twice then fill it by yourself according to your documents and after that again read and check it twice and then sign the form and attach appendix8. Enclosed all ORIGNAL documents with TWO sets of photocopies. Keep one extra set of photocopies of form and documents with you for your record.
6. Get appointment at your nearest UK visa application centre, pay the required visa fee, attend interview if required and wait for the decision.