Strength Training and HIIT Questions

Hi! So I started doing hiit recently but I am extremely new to weight lifting/ strength training etc. With school coming up I know ill be pretty busy so im thinking of doing my hiit workouts 3x a week and strength training 3x a week so that they overlap with one rest day. Is this enough? Too much? also what are good beginner strength workouts? or helplful links etc. I am looking to flatten my belly and tone up a bit. I have really big, strong shoulders compared to my body so that is not something I want to make bigger. sorry im just really new to all this.

BTW: I eat a balanced diet so that is nothing to worry about. I am 16yrs old, 5'6, 120lb, and eat around 1400 cals a day.

Any help at all is much appreciated!! xx