I've heard more repetitions at lower weights will allow me to get lean vs building bulk. Should weight training be a relatively small part of my fitness program? Should I focus on running, swimming, biking, other cardio type of workouts? I do not have very defined muscles to begin with.

The last 6 years my activity levels have dropped down due to work and university and I'd like to get fit before it catches up to me. I am a training noob as I have always preferred to play sports or other activities vs training or gym time. So even simple tips that may seem obvious would be of help to me.