How do I make my glasses look realistic?

Just use the principled shader, set the transmission to 1, roughness and transmission roughness to 0 and make the base color pure white

Use this node setup to get realistic glass shader in cycles...

If you add in just a glass shader, blender will try to calculate all the caustics which in turn will fill you scene with a lot of noise.The node setup I have mentioned will fake the object as a transparent yet shining surface..

You can have a basic glass material if you mix these 3 nodes: Glass, Glossy and Transparent. For example this seems to work fine:

You can select the mesh of the glass and give it a Glass BSDF material. Changing the roughness value in the properties should give you the amount of transparency you want.

No Nodes Necessary.