moving in year 9

I have to change schools near the end of year 9 or start of year 10 but I am worried this will affect my learning. Will I be behind the rest of my classmates and will it decrease the chances of me getting the gcse subjects I want??

I cannot say for certain no.

I would say that it would probably be worth getting in some good revision in before you do move, so you have a good foundation of knowledge before changing school. I don't know how options work at the school you are moving to work, but if you are already in contact with them you may be able to get the options you want (sooner rather than later).