I RECIEVED A LETTER TO BE A JUROR..WHAT AM I TO EXPECT,THIS IS MY FIRST TIME HAVING TO DO THIS.?

MY NUMBER IS 195..CAN SOME TELL ME HOW IS THIS DESIGNED and what do i have to look forward to. i hope im not the only one who's in there and is clueless about what to do,etc. how does it work.! do a number of people get picked? what happens if you get there and they dont need you? im scared and on top of that im clueless..but i have to go in tomorrow at 8am.