You will need to use the "grep" command. For example if you are looking for "Hello there" and all the files are in the same directory, try:
grep "Hello there" *
If you don't know in which directory the file is, you will also need the "find" and "xargs" commands. First you will need to change to the directory that will be starting point of your search and then type:
find . -print|xargs grep "Hello there"
or
find . -print -exec grep "Hello there" {} /dev/null \;