I've been to: BB17 launch and 2nd live show (when they swapped housemates between the two houses) and on BB18 launch.
I recommend reading my Post about it:
http://www.thisisbigbrother.com/foru...d.php?t=302464
It's basically divided into regular tickets and priority (people with paid membership at ApplauseStore).
Considering you're going on the regular list, I would recommend getting there around 3PM at most to guarantee your entrance (I know it sounds early, but this is how it goes on launch shows + being CBB, many people arrive and they can't let everyone in, especially if many priority tickets' holders show up).
You'll have plenty of time outside, so bring some music/book/something to watch while waiting, and you can always pop up to Tesco to kill some time (and buy food/drink, but please note that you can't bring them inside the studio).
If you have any more questions, feel free to ask.