Sysadmin to set up the server. Urgent!

There is a server with about 200 sites. Recently, some site or sites have become a big burden on the server. And from time to time, the server is either very slow or not available at all. I tried to dig into Mysql, apache, nGinx myself - I never figured out what it was like :

My earnings depend on the server, I will thank you for the quick and high-quality solution of the issue!