Day 1 is Strength Day.

If you have a Strength routine, go ahead and crush it.
If you aren't sure, perhaps follow Strength 1, 2, 3, or 4.
Also, if you are at a gym, just use day 1 to get familiar. Do a few sets with various free weights (dumbells), stack machines, and plate loaded machines. Sort of figure out your weights, what exercises you like, etc.

Remember, Resistance training/Strength days are slower lifting. Keep reps at about 8-10 reps. Work to "exhaustion" (reps 6-8 should be hard burning) but not to "failure" (can't even get the 8th rep). Rest about 2 mins between sets (recover for the next "lift").

Btw, warm up 10-15 min. cardio is fine.

Those days are key to fitness and weight loss.

1) Controlling diet and intake is the only thing that matters in leaning down. Correct!
2) So with that in place, some nice resistance/strength training is the best, safest complement to a seriously controlled diet.