What can I do to reduce server load with phpBB3? My forum isn't that big yet but I want to prepare for the future since I'm only on shared hosting.
Worry about that later imho. First see if you forums does well. Otherwise you may be just wasting your time.
how do you know abot that load? uncoment lines in botom in config and then will show you queries in footer
Don't enable gzip compression, it saves bandwidht but uses more CPU resources. phpBB3 doesn't use many system resources, it's very lightweight.