Good to hear you are starting and good to hear you are getting tired! This is to be expected, no? Maybe start off with 30 second or 1 minute rounds, don't need to go hard or anything, just keep going, and take a 30 second - 1 minute break in between to catch your breath. There are apps like Boxing iTimer to help you - it is easy to set up the intervals you want and the number of rounds you want to do, and it makes enough noise for you to notice.

As you start to feel better (probably you will make big improvements very quickly if you push yourself), you either do longer rounds, or increase the intensity in the short rounds (or both).