go to your options tab
and click advanced and change everything to the lowest quality ( you can hardly tell the difference between highest and lowest)
that will improve your fps by about 30-50 and get game booster
just go on google and type "game booster" and the first link is it.
game booster just disable all the useless programs that run in the background (you can enable it or disable it)
or you can go to the computer store a buy a new computer