There are lots of things you can do, but the general process should be this:
1) learn the content (this could include things like watching youtube videos, using a textbook, making flash-cards, mindmaps, etc.)
2) test yourself (past papers, end of chapter questions, etc.)
3) evaluate how you did, and find ways to improve (use a mark-scheme to check your answers, ask a teacher if you are stuck, look up the answer online, etc.)
A good YouTube channel is called Get To Know Science. A good textbook would be the one made by OCR, or CGP. Use the OCR website for past papers.