Normal Map Help

So ive recently attempted to make a model of a lee enfield 303 for a small game i would like to make. I made a small low poly model then adding the subsurface modifer. Problem is for me now is there is too many verts and So unity has a hard time using that. So ive tried to make a normal map for it but now am stuck. The normal map has not been working and am not entierly sure if I am doing it correctly. So if anyone can help with this it would be appreciated. I would really love to get back to work with this but am just stumped with the normal maps and trying to get a model that looks as good as the high poly model but can be used in a game engine