I'm very new to unix
I have a checked project , there exists a file called xxx.config .
now my task is to find all the files in the checked out project which references to this xxx.config file.
how do i use grep or find command .
You almost certainly want to find out about the man pages. They will give you documentation on how to run most standard commands on Unix, including grep.
Take a look at that page, and try to work out what a suitable grep command would be - we can then help you from that point onwards. You should not need the find command (unless this is a school project (for some reason, some schools insist on using the two in combination, even though it is redundant and bad practice)).