This is a huge subject and runs the full gambit from which hardware and operating system are installed, whether or not VM is being used or not, how SQL Server was installed and what settings it has been setup with, how the hard-disks have been formatted and partitioned, how the databases has been setup for growth, how the tables are designed, how the indexes are designed, how the queries are written, how DB maintenance is done, etc, etc, etc and even includes the number of sun spots and the position of the moon. 😉

My recommendation is to take Step 1 and get some training if you're going to do the job of a DBA. If that's out of the question, consult with my personal "Assistant DBA"... His name is "Google". Tell him you want information on "steps to take when DB running slow". In the process, you'll also learn, which takes us back to Step 1.