All the plugs in my house only put out 51 volts why and how do I fix?

Hmm... do your lights and appliances work? If so you need a new voltmeter or instructions as to how to use the one you have.

Some things work but I'm trying to add some lights that's take 120volts and they won't start up

Agree with MB. If things are working in the house, you are either reading the voltage wrong or the VM is bad.

I would Start Measuring at your Service Disconnect, then your Main Panel, Then out of each breaker.
Your Meter could be bad, test on a neighbors house?

If you have a whole series (several circuits) out, you may have a problem with incoming power.

Did the failure start when you started adding the lights or was this a pre-existing failure?

How are you reading the voltage? Do you have an Analog meter or a Digital meter?