Go to Settings in top bar of Cura, Click "printer", Go to "manage printers", Go to "machine settings", then click the "heated bed" box.
In case you are wondering how I know this, I asked the same question about a year ago! Don't be afraid to ask questions, we all had to start somewhere and 3D printing can be overwhelming at times and the simplest things that seem monumental stumbling blocks to a beginner are easily addressed by those of us that have been where you are now and are willing to help.