If you lifted the dumbbell 8 times, you did 8 repetitions. You should really concentrate on your form at first and not worry so much about how much weight. A sample guideline would be if you can do 12-15 repetitions and not really feel much resistance, increase the weight a little bit. If you can barely do 8 repetitions, decrease the weight a little bit. If you are really strapped for time, you can do a modified form of the above workout that only includes compound exercises.
Compound exercises work many different muscles at once.
An example would be the Dumbbell Squat . Some of the muscles that it works are the buttocks, thigh, and hamstrings. A Tricep Pushdown would primarily just work the triceps, so it would not be considered compound. You can also add some Crunches for your stomach and some Calf Raises for your lower leg. Try doing either of these workouts 2-3 times a week and not on consecutive days.