Hey I am an A level biology student. I would suggest that you try to understand the concepts behind each of the topics, and then learn to answers exam specific questions.
1)buy a revision guide from CGP which was made for your exam board and level of study.
2) Watch some biology related videos from bozman science on youtube, on each of the topics that are in the textbook to try and get your head around the concepts. For example there may be topics on evolution, the heart, ext.
3)read though the revision guide and take notes of things which look important, my advice would be for GCSE to condense the page down to about 5 lines.
4)practice exams can be found online, I don't know what your exam board is, but if you search past papers for your exam board you will find them. Your teacher will show you how to get past papers up if you ask. Do past papers then go though them checking your answers, continue this until you achieve the grade you want.
5) A concept may come up which you don't understand, or can't seem to get your head around, in this instance consult a teacher. For example you may simply not understand why geographical isolation can cause evolution.