Awesome, you are off to a great start! It's great to know you are so young yet so on track with yourself and making this healthy lifestyle change. Lord knows I should have started at your age.
The run/walk intervals will get so much easier for you the more you do it! I started running for 90 secs and walking for 90 secs for a total of 20 minutes and as of yesterday I pushed through two sets of 10 minute runs. It's taken me 7 weeks to get here. You can do it soon!