Regardless of your fitness level, you should choose dumbbells with which you can do your exercises while maintaining proper form: good technique is always more important than number of reps. At the end of your workout, you'll feel that your muscles got a good workout.
When choosing your weight, you'll need to consider the muscles you want to work: opt for a weight of 2 kg and up for lower body exercises, and divide the weight in half for upper body work.