I have 5 lb dumbbells but I really believe(well, I know) I could do way heavier. Anyway, what do you guys recommend for starting dumbbells? 10 lb? 20 lb? 30? I want to build muscle at a medium-fast pace and I think 5 just won't do that. Sorry, I'm a beginner with at-home weight training.

If you have 5s, I would add 10s. 30 is really heavy for dumbbells, the most I use for anything is 25s and it took me a year up to using them at all.

You'll find that different weights will be needed for different exercises. For example, I curl 25's. I can press 20's. I can do tricep extensions with 20's. But for shoulders? So weak. On lateral extensions (where you raise the dumbbells from your hips to shoulder height holding your arms out sideways) I can only do about 5 lbs in each hand. And if you're doing a program that focuses on lots of reps, the weight you can do goes down as your reps go up. A set that goes up to 25 lbs would probably be good for quite a while. I got a set of those Bowflex Selectechs this year and love them- one set of dumbbells replaces them all.