I started a cutting plan a month ago and I've followed it nearly to the T! And I haven't lost a single pound and I can't tell a difference in the way my body looks. Maybe it takes more time but this is frustrating!

You need to provide some specifics about your training and diet, how long you have been training and your training history, your natural build and body composition etc for anyone to be able to give you a decent response.

Ok, I am 18 years old and am 6'4". I weigh 215 pounds. I have little muscle so the weight is mostly fat. 3 days a week, I run high-intensity intervals for 15 minutes. The other 3 days are lifting (days of chest (bench press), legs (squats), arms/shoulders (dumbbell stuff), and one day of rest. I have also cut back my calories to around ~1900 per day when I was getting around 3000 before. And the calories I eat now are healthier (oatmeal in the morning, chicken breasts, veggies, etc) with small amounts of junk food whereas before it was all junk food. I don't understand what's wrong?

I'm trying to learn something about health and fitness here, so don't take my advice to quickly (or at all?) but I'll see if I can get it to you before someone else does (assuming I'm correct).

1900 calories from 3000?

I do beleive you should drop 300 calories per week (unless you did, and this is how far you are now). Additionally, you should be getting more calories then that. Your height and weight definitely needs more calories.......especially if your doing all that exercise...

Oh well, we'll see if I'm wrong

Did you cut your calories gradually? If you do a drastic drop your body kicks the fat storage up the priority list. You won't lose weight do to caloric defecit that way, you'll lose muscle till your body reaches a weight that it can be efficient at again.

..That would probably be it. I went right to 1900 from 3000. What's the best way to kickstart my metabolism without gaining weight?

Ok, I am 18 years old and am 6'4". I weigh 215 pounds. I have little muscle so the weight is mostly fat. 3 days a week, I run high-intensity intervals for 15 minutes.
--------------------------------------
ok i m not much in favor of HIIT, dunno why but it just doesn't suit me... anyways i never looked HIIT in much better way...
i prefer good ol ways of doing cardio
like Running, swimming, shadow boxing...

do it for at least 20 minutes(start with 10 minutes round)
then gradually build up to 20-25 minutes

u need to understand that for weightloss/fat loss u need to work out in specific range of Heart rate(which is about 75%-80% of ur Maximum Heart Rate).
ur Maximum heart Rate(Roughly) would be
220-urage

if u r not worried about muscles then do it empty stomach in the morning.
WhoAmI