How to improve a storedprocedure / sql statement on multiple sessions

Hello I am new to SQL Server . I’ve used idera diagnostic software to evaluate the performance of my storedprocedure and sql statement and i’ve found out that as i increase the number of users connected on my server, some of my stored procedures tend to have a higher duration greater than 5000 ms in which in a normal situation when there is only 1 user, usually it will not took < 20 ms. Even a simple select statement would take 10000+ ms ( if the server is busy ). . . Any idea why these things happend? or any suggest as to how can i improve ( if there's a way ) . . . considering I am using two database.1st was used 4 testing (getting the server busy).while the other is for performance evaluation which i profile.