To search in Vim just press (apple-f for macvim) / then search string.
/search
This will highlight all occurrences of 'search', to clear the highlighting enter:
:let @/ = ""
To search and replace the first occurrence on this line only:
:s/search/replace/
Lines 1 to 10 all instance per line (thats the /g global):
:1,10 s/search/replace/g
All Lines All instances
:%s/search/replace/g