Should I join CS major at college or start building my own startup?

That piece of paper makes all the difference. Hang in there for the last year and you will be happy you did. It is tough to land a good job without a degree.

Good luck with whatever you choose.

I'm in a very similar position myself. I'm about to enter my 3rd year at UT Austin, with a double major in Business (MIS) and Computer Science. I have gotten offers to co-found 1 or 2 startup ideas with adult founders that have had very successful companies in the past (that they started!). I was originally only MIS, but my interest has always been in tech tinkering so CS was a must.

I've talked to tons of people and asked this same question and I've learned that a CS degree is VERY useful. Besides engineering, it's one of the most valuable degrees you can get. Many companies don't care about you grades if you're in CS / Engineering, they care about your experience, intelligence, etc. I would strongly recommend working on a startup idea on the side and/or getting an internship in the semester (if no startup) or a full-time internship next summer. Grades are important, but honestly if you have around a 3.0 - 3.2 and you're doing a technical field and you got good startup / internship experience, you're much better than a 4.0 CS guy with very little experience, hands down.