To find a file use find:
find . -name *.html
This command would search from the path you are at the moment (that is there is a ".", but it could be any path). The "-name" would search for a patron in a name. So the *.html means that we would search for all files ending with .html .
The grep command is use to pick a line with an specifc patron. For example you can search with grep using a pipe from ls (I guess you try it)
ls -R path | grep .html
The "path" could be any path.