I am a fresher what things I should know before joining a startup full time ?

Depends on startup size & user base.

One thing common for most*

• Embrace yourself for hard work. Putting everything into it.

• Prepare yourself for self-doubt and speed. Make sure you work extra to cope.

• Always remember who is helping you, and how you can repay them. For instance, if you are asking for a session of 1 hour with your mentor, make sure you reduce his work by 2 hours later on.

• Everyone is short on time, so make sure you are well prepared before meetings, make sure you ask the right questions & respect everyone's time. If you do, they'll be happy to help you in future as well, if not, well they'll just avoid you.

• You gonna work harder (and much more) than your batchmates. You choose the path, live it, trust me you gonna be much better than them down the line.

• Again, be quick, smart and intelligent. That's the only way to survive, except being dumb, extremely hardworking and submissive.
  How to be quick, smart & intelligent? Simple, by being dumb & hardworking.

• Adapt quickly. For everything, everywhere, every time.

• Observe everything around you, don't be too frank when you are starting. Observe people. Their work. How they communicate, what they communicate.

• Be very kind & humble to everyone. Gonna help you a lot.

• Take all criticism positively.

• Don't restrict yourself till work, learn how it's impacting the business,
  why this, why that(super most important thing)

• Ask questions, questions, questions, about everything, every time.

Most importantly take time for yourself & the relationships you have. Please, please. It's very easy to not stay in touch. But preserve those relationships. After all "You are replaceable for your employer, but not for people around you" And that also includes you, create a nice relationship with yourself!

Have Fun!.
Enjoy your time, kal ho na ho? :)

It's been mentioned in the above comment,but know that you won't have any life. And , it's part of the hustle.