If a microwave runs but doesn't get hot then this can be a number of things what can cause this.
Either the magnetron, diode, transformer, high voltage capacitor or internal fuse.
But to check these you will need to remove the microwave and take the cover off. Now microwaves can hold a very high charge of up to 4500v even when unplugged. So before touching anything you need to discharge the high voltage capacitor first.
From there look on the circuit boards and see if theres a small fuse. If so test this and see what you find. If faulty then it will need to be replaced.
Im afraid other then that you need a good multimeter to be able to test diodes, capacitor. So microwaves are not the easyist thing to fix yourself unless you know how to test them sorry.
But a lot of the parts fitted are very expensive. You are looking at about £100+ for the magnetron, transformer as these parts are expensive.
If you need anymore info just get back to me.