how to find out which part i need?

That is an impossible question for anyone to answer from the other side of the internet. It could be anything. Broken wire, faulty element, faulty thermostat, faulty selector switch. You need someone with a test meter to check each part and find which is faulty otherwise you will just be wasting money buying parts and then finding it still doesn't heat.

Lincoln Appliance NZ
June 2014

Most likely the round heating element behind the backplate inside the oven.

Kenjo
June 2014