Basic information is always good to post when asking questions on forums.
seeing that we have no idea what boat and motor you have , what skill level you have, what tools you have , what tach you plan on installing, and what you will have to cut through in the dash, it is not an easy task to answer your simple question.
There are tach kits available for your motor I am sure that come with the harnesses needed to install it, but we have no idea if your motor has a working trim sending unit to hook it all up to