Resistance training is important.
When you lose weight, some of that is
fat and some is muscle. If you don't work to keep that muscle mass, you may end up what gets called "skinny
fat", thin but without any muscle, and possibly even high in
body fat %.
Resistance training helps you keep that muscle, so that as your weight goes down, so does your level of
body fat. This is how you are going to get that lean look.
You will still lose weight doing just
cardio (as long as your
diet is on track), but if you want to look good, and be as healthy as possible, you definitely want to add resistance training.