What does Dartmouth look for in terms of EC’s? Do they like to see you participate in all sorts of different clubs and teams through all four years? Or do a few and be leaders of them? Also, how much to they value you founding a club?
They’re not as important as grades, test scores, LORs, essays, etc.
Some would say that ECs are arguably the most important factor in your whole application after only your grades. Maybe it would go grades > ECs and scores > essays > LORs.
Dartmouth, like most other selective schools, tend to accept students who display a “spike” in their application. This means that the student has demonstrated they are passionate about a subject matter through a bunch of similar ECs, awards, and classes taken. Be an ace in one trade rather than a jack of several trades. For example, if you are interested in writing you could join the school newspaper, join the town newspaper, write for a literary magazine, edit for a literary magazine, write a novel, write a short story, publish a book of poetry, win Scholastic writing awards, etc. The key point is to do several of these to show that you are skilled and passionate about an academic interest.
Be a leader of one or two clubs. Don’t join everything possible.
Clubs are easy to start so I would be surprised that an AO would value starting a club more than holding a leadership position in an established club. In fact, I think being chosen as a leader in an established club - and showing some progress in the clubs results - might be more beneficial as it shows that other people respect your leadership qualities.