Well it's time to go to the gym, it's going to be my first HIIT program I've ever done, hoping the short, intense, exercises will help me gain some good muscle mass
It'll help you burn stubborn fat, not so much muscle mass. Good luck though.
No one showed so I just hit the machines
HIIT (or maybe it's the wrong term, I'm not sure, I'm also calling it this way) is also great for building mass:
Get to one machine you like to do, put on a weight you can handle with about 90% of power needed and where you are able to do about 10-15 reps. Hit it until your muscle is sore. Then - with no pause in between! - reduce the weight by about 5-10 kgs and immediately do another set with as many reps you can. Then reduce the weight again, again as many as you can. Repeat it until you end up with 5 or 10 kgs and your muscle feeling like exploding - there you have your HIIT muscle program.
You can do it with whatever machine you like. Just make sure to do this on the end of your session and don't torture this specific muscle with another exercise. Also remember to have a rest day (for this muscle) the day after.
Most important: aim for the most clean movement possible - let a trainer probably show you the best execution before - and keep it until the whole session! Don't get sloppy even under highest impact: HIIT (with this muscle method specifically) is very likely to cause serious injury when not done right!