I am very skinny all over. I understand what you mean about being disporoportion if my arms get big, but right now I just want to get my arms 'normal'.
I need tips on curling, how I know when to stop, to push more, etc.
First, I have to try one more time: if you are skinny all over, the best bang for the buck is a full body strength training routine. The gold standard is the routine in
Starting Strength. The book is also the best resource I've seen for teaching all the basic lifts (curls included).
Stronglifts 5x5 is a similar routine and the website is a decent free resource. These routines take ~60 minutes per day, 3 days per week, and the results you get from them will be INCOMPARABLY better than playing with 40lb dumbbells at home.
Second, even if just getting big arms is your goal, isolation exercises (that work one muscle/joint at a time) like bicep curls are not the best choice. Compound upper body exercises (which work multiple joints and muscles at the same time) will build size and strength in your arms much faster. There are two major muscles in the upper part of the arm - the biceps and triceps. To work the biceps, do pulling exercises, such as pull-ups, chin-ups, rows, and inverted rows. To work the triceps, do pushing exercises, such as push-ups, dips, and bench press.
Third, whichever exercises you end up doing, beginners tend to do well with sticking to 3-5 sets of 3-5 reps per exercise at some set weight X. If you were able to complete all reps and sets for an exercise, use X+5 at the next workout. The initial value of X for the first few workouts should always be light so you can focus on technique. As the weight increases, so will your strength. If you eat a caloric surplus, you'll add muscle mass too.
Finally, there is nothing complicated about curls. You can do many different types (see
here) and the key is to avoid swinging your body so your biceps are doing the actual work.