As someone who works in the legal field this is how it goes in our system:
Everyone called for jury service will be told to come to a meeting place (there's usually a jury service room in the court or in our case they are currently using a local hotel) Names are selected at random and you will be called into the courtroom. However, solicitors are allowed to challenge people, if you are challenged you won't be part of the jury. But that doesn't mean your name won't get called for another case!
I'm sure you already know these rules but:
You cannot be part of the jury if you know anyone involved in the case or if you have heard about it/know what's going on
You can't discuss the case with friends/family
You aren't supposed to research the case online or on social media for information relating to it
In terms of breaks in our system you are allowed to leave the court and will be told when to come back. Food was also provided in the local hotel for jury members.
There is no set time for the duration of your service as it depends on the case. It could be 2 days or 2 weeks, it's never set in stone and it doesn't always follow a 9-5 schedule. Some days you could be finished at 2pm and others late at night but if you have an early finish on certain days you won't go back to work that day. Your employer must allow for however long it takes. I have seen cases where the jury was empanelled in the morning and the accused pleaded guilty in the afternoon - so the jury was excused without having to do anything and their service was complete.
Finally it's nothing to be anxious about so don't be worrying yourself. You may not even be selected but if you are a lot of people actually find it fairly interesting. Make sure to have something to keep you occupied like a book as the others have said and pay attention to what is going on as coming to a verdict is very important.
Good luck!