selective subsurf?

i want to make a subsurf in a part of mesh, but not to all cuz that would increase the verticies by a lot, so how can i apply a subsruf to only selected verticies?

you can’t, but you can achieve the same in a different way,
separate the parts- and then apply subsurf to that specific part, that is the simple way.
if the parts are connected to each other and you can’t separate them - then you have to add it to the whole mesh and use loopcuts for the parts that have to be sharpened.