how can I upgrade if my mac is early 2008

Well, it depends on which OS is on it now. If this is yours:

https://everymac.com/systems/apple/macbook/specs/macbook-core-2-duo-2.4-white-13-early-2008-penryn-specs.html

then the highest it can go to is 10.7.5 which is Lion and is from 2010/2011. You cannot go any higher. And remember, you have a completely outdated machine, both hardware and software, which is 13 years old. I am amazed it is still going - count your blessings and start saving for a somewhat newer model. Jump up to one at least 2016/17 or so.