Hi Lukas, welcome to our board
My question is how much weight I should use when performing certain excersises, especially dumbbell and barbell excercises.
When starting, use a weight you can do for 15 reps. The last rep should be hard, but still possible with strict form, so that you feel you could do a few more reps if you pushed really hard.
The reason why no exact weights are indicated is that this is very individual. Obviously it will differ a lot for different persons. You have to find out how much weight you can use for a certain number of reps.
Note that it doesn't include any weight suggestions. I'm limited to my 2 x 4 kgs (hence my previous topic hehe) dumbbells I still have, but I don't know if that will cut it.
4kg is little weight for dumbell bench press, even for beginners, unless you have some health issues or are not young anymore.
You should get yourself adjustable dumbells and/or barbell with plates. Work out regularly and aim to increase the weights. After a few months, you can also start to decrease the number of reps per set while increasing the weights.
How do repetition and the weight which you repeat with relate to one and other? My personal goal is increase of musclemass and perhaps weight gain, so which aproach would best suit that?
For mass, i believe variety in the wide rep range of 5-15(-25) reps is best. All depends on you: your routine, your preferances, your disposition etc.
Lower reps will train strength primarily. Strength and mass go hand in hand, however.
As a beginner, concentrate on learning proper form on some main movements and focus on increasing the weights on those steadily.