If you have GeForce for Nvidia. There is a way to automatically optimize ArmA per your computer specs that the app reads.
Now if you dont, you are going to need to manually adjust to find the best settings. Keep in mind that performance/frames will be severely different from you in a server by yourself compared to 60 troops in a server.
There are suggestions on youtube/internet but everybody's rig is different so tweaking will be in order.