For startups the main thing is culture. Ensure you adjust to the startup culture as somethings may seem off from the start(more informal, chill, etc).
Startups move fast, so it will be hard getting used to the change at first but later on in, you will adjust and move just as fast. Of course you have to put in the extra work from the start to adjust.
Be prepared to work long hours and on weekends. While not necessarily true for all startups, it is quite common.
Get to know your team. Hang out with them, grab drinks, play board/video games, etc. It will make communication much easier.
Always look for ways to improve the company and offer suggestions. Sometimes, due to other work people will acknowledge your ideas. I personally used to go the extra mile put in the extra work and time and make the fixes/changes/build new features myself.
All in all...have fun and learn lots!