I would like to install a switch for a pull light fixture. The only power is from the light fixture, but I want to put a light switch on the wall as I enter the room, Can you help ?

You need to be able to fish wires from ceiling light location down to the switch.
If Romex, tape the white at both ends. Connect the 2 wires to the switch, and connect the ground.
At the light, with power off, remove the switch, connect the romex to where the old pull switch was.
Connect the white with black tape to the wire that is constantly hot.
black to the other connection.
If fishing the wires in not an option, X10 has a module that goes in the fixture, then you need the wireless switch that has double stick tape and can be placed anywhere, you can have a couple switches if you want. No limit.