To use your request as an example:
find / -name notes.txt 2>/dev/null
The first param ie '/' represents the dir to start from; find automatically recurses down through all sub-dirs from it's starting point, so don't use '/' unless you really are lost ...
the last bit ie '2>/dev/null' sends stderr (ie error msgs) to null, basically throws them away.
Very handy, particularly if you start from '/' as you'll get a lot of these, even as root user...
You can also change '-name' to '-iname' which means match filename case-insensitive.
If you want to use a wildcard eg *.txt, enclose in single quote marks to get the desired effect eg
find . -name '*.txt' 2>/dev/null
to read up on this cmd.