If you are in 8th class now, and studied everything very well in previous classes, there is no need to worry. U will pass.
In case you didn't study enough in recent years, just buy any book ( Upkar is well known, or Arihant). check the syllabus and the type or pattern of questions in last year test (mock tests). Then prepare the topics from regular books or the books you used to read in school. After clearing every topic , do practice and practice only. You have enough time from now.
Moreover, you could also look for NCERT textbooks of class 8th. They'll help you out.
All The Best.