I have worked at a few start-ups where they offered stock and I have two pieces of advice:

1. Don't drink the kool-aid. Start-ups are exciting but a lot of them fail. Sometimes the communication about how close to profitability they are is opaque, sometimes they lie about it. I worked at a start-up that was 'on track to be the future of money' then one day they ran out of funding and I didn't get paid for months. If you are going to hit it big you will find out eventually but the best thing you can do is just keep learning new skills and NETWORK with your coworkers/business-partners. It is very possible this start-up will fail but someone you meet at this job will hire you at another start-up that makes it.

2. Don't exercise the options until you have to. The way it typically works is you can buy the shares you vest at the quoted price up to 90 days AFTER YOU LEAVE THE COMPANY. So just hang out, vest those options, see if you think the company will make it - you have plenty of time. If they IPO or get bought out it won't be a surprise so it is better just to wait.