@ sanam
Gen sci is all about the science in general. U need to build the basic concepts of it and for this u have to develop a curious outlook at the things around 1st. Going through books is nonetheless important, but the other half to understand, is by discussing ur querries with the people who have the basic concepts of it. I think that the august members of the forum are also here to answer with interest if u have any queries. It would be a help for us also, in the manner that it wont only make our expression of thought better, but it is not beyond possible that u may bring into our consideration any topic about which we also dont know. Then i think we will search for it together.
Also keep in view the way u r required to reproduce ur answers in the paper and build ur concepts, accordingly. For this reason, i think that Islamabadian has already suggested u a good book.
For other references, u can consult the internet also, try to start with the encyclopaedia and then gradually move onto comparatively complex form.Googling is a good strategy to hunt for the answers.
http://www.howstuffworks.com
is also a useful site to help u deal with the basic concepts of everyday science topics. Stress on the concepts than cramming up.
Remember, only a good strategy is the shortcut to hardwork.
All the best !