Heart rate monitor/calories burned question.

Hi thanks for reading! I recently started working out, well for the last 2 weeks. I started the butt bible workout on youtube and its going well. I have been trying to eat the same as usual yet a little more and i have upped my protein intake. But really my aim is to eat enough back that i am burning as i dont want to lose too much fat. My brother has a heart rate monitor and i synced it up to an app on my phone tonight to have a go with it whilst doing nothing and during that 10 minutes i had burned off 26 calories so im guessing its calculating general calories burned too at the same time.

So anyways I just need some help in knowing how to calculate things as i imagine i work out more my metabolism will increase and more calories will be needed in general when not working out. Also there's calories that would have been burned anyway normally at rest like i was testing out earlier. How do i calculate this all? I cant really get my head around it so will ask for some help on here. Thankyou for any replies Much appreciated!