There's plenty of videos on YouTube covering most,if not all of the A-level content, regardless of the exam board. Which exam board are you doing?
Search on Google for the specification that you need. It will be the new specifications because they are all changing and it will affect you as you're in year 10. There are 6 modules in total and 4 compulsory modules core 1-4.
Then you pick 2 applied modules out of mechanics 1-2, statistics 1-2 and decision 1-2. It have to do C1-4 but you can pick any combination of the applied modules.
To start A-level maths you must have a good understanding of some of the basic concepts - algebra, surds, solving quadratics, completing the square. Without these concepts it will be impossible to do A-level maths. Have you done any GCSE maths papers or have you got a grade in GCSE maths? I ask this because some people think that it will benefit them if they do A levels in year 11 - but it doesn't. Some unis and some courses require you to have done your A levels only 2 years - so year 12 and 13. So if you are thinking you may want to do an a level in maths before year 12 it may not be a good idea.
If you cannot do GCSE maths and get an A* every time and get pretty much 100% (dropping maybe 1 or 2 marks only from mistakes and not understanding) then you should definitely not start doing A-level.