Hi Lydia
I went to the show last Saturday and, like you, I had very little idea what to expect or what to do.
Some very kind forum-ites provided some info and pointed me in the direction of the "What to expect...." thread (which was on page 4 last week). I was advised to get there between 12 and 1pm as the queuing starts early and the volume of people in the queue seems to vary week to week. Anyway, this is how my day went:
Arrived at 12pm (via taxi - v helpful driver who pointed out the Audience Reception to us). We were about 17th/18th in the queue behind some seriously prepared people with chairs, stools, rugs and flasks, and (it later turned out) 8 Ronan Keating fans who had managed to get SCD tickets but weren't interested in the dancing

(don't get me started).
Next in the queue behind us was DancingPixie & friends who gave us loads of valuable advice and toldus what would happen inside.
The time passed fairly quickly as everyone was chatting and gawping at the Ronan fans (who ran at him both times he came in/out of Television Centre).
We queued til 3:45 then the doors opened but a separate queue for production/staff guests was organised so there were two queues (one in each door).
You have to put your bags and coats through the security machine like at the airport and are checked to make sure you aren't trying to smuggle anything dangerous in!
From there you go across to the canteen, collecting a numbered sticker for your ticket on your way.
We stayed in the canteen til 4:45 watching old SCD re-runs. Ian Waite was in the shop signing DVD's/tickets/books.
We tried to queue up at the studio door early (it does help to sit at the top near the door!) but we got told off as it was 'strict number order'. We were lucky as we had 26/27. They called 1-30 then 31-60 and so on.
We were taken out to the studio and told where to sit. they filled up the spaces at the back of the dancefloor first, then at the sides and upstairs.
A warm-up type guy came on 15-20 mins before to tell us what was going on then Bruce came out 5 mins before.
After the 1st show we got the opportunity to quickly get on the dancefloor to meet some of the professionals, even though the BBC staff tell you not to, they don't stop you.
I met Brendan and Anton, got autographs and photos. They were both lovely.
We went back to the canteen for 45 mins then came back out for the Results Show dress rehearsal. They filmed the Ronan performance, ran through the results (with a fake elimination) and the professionals danced the salsa.
After the Results Show it was more difficult to meet celebs/professionals as the dancefloor is crowded with audience/friends/family and also the cloakroom shuts 15 mins after the performance so you need to get your coats back.
Have a fabulous time - we did!