Without knowing what topics you read or you tried, there isn’t much way to know what to tell you. The easiest is to use two separate servers. The hardest is to try the subfolder (example.com/forum).
Did you follow any of these? Use a subfolder (path prefix) to serve Discourse with multiple servers sharing a domain. Run other websites on the same machine as Discourse - #47, Set up Discourse on a server with existing Apache sites