Go to your desktop and right click, then go to Graphics Properties, then go to 3D and set everything on it's lowest possible option.
You should try reducing lag by going ingame, pressing Esc and opening options, turning water reflection off and lowering view of distance.
You can also try cleaning your disk, get rid of all the useless stuffz. (You can use CCleaner for that)
If you don't know how to, I'm sure there are plenty of tutorials on youtube that can help you. c:
Sorry if this was confusing in any way, derp.