With the information you have supplied you should not expect an answer. Do you have a laptop or a desktop? It sounds like a laptop--if so, you should go the web site of the manufacturer of your laptop and join a forum there and ask your questions there--and you will no doubt be assisted by other people who have bought the same machine you own and can guide you. If, however, you have a desktop, then you need to post what kind of desktop it is, processor, etc.
In most dual-GPU laptops, the smaller, less powerful GPU is used for power-saving purposes, but when you do something demanding like start up a 3d game, at that point the more powerful GPU should automatically activate. Or, you may have to turn it on via software. If it's a laptop, your best resource for help will be the manufacturer's web site.