You'll have to fish a switch loop to the switch location. Also get a new fixture that doesn't have a pull chain.
- Remove existing fixture from the ceiling.
- You (hopefully) have one black wire and one white wire. (if not come back before you do anything else or remove any wires from the fixture).
Now it's fishing time, unless of course you're lucky and the walls aren't finished. You have to get a piece of 14/2 NM from the light fixture mounting box to the switch box. Done? Good
IN SWITCH BOX
- Connect one wire to one screw, the other to the other. Doesn't matter which wire goes to which screw.
- Install switch in box.
IN LIGHT BOX
- Wrap black tape around the white wire to re-identify it as a hot wire.
- Wire-nut the new black (old white) wire to the original black wire.
- Wire-nut the black wire (has always been black) to the black wire on the new (no chain) fixture. (their might be screws on the fixture instead in which case you would connect the wires to them, clockwise)
- Connect, by whatever means is applicable, the original white wire to the white wire (or silver screw) on the new fixture.
- Install fixture onto box.
Turn on the power and test. Celebrate a job well done. Be happy.
BTW, since you have young eyes watching your every move....no shortcuts. Take every safety precaution, even if you think it's a silly one.
Turn off power at the breaker. Turning off the switch isn't enough. After the light is free from the ceiling and before you touch anything, retest for power with a simple test light or non-contact tester.