Ah bless you, I completely understand. Coming back to work after a break is always hard as is starting in a new office! You'll be fine though. I think the number 1 thing to remember is that in work everyone tends to be preoccupied with their own stuff and so as long as you are polite and pleasant, no matter how much you internally feel super nervous and awkward it's unlikely people will really notice or care much (in the nicest possible way!). If they do notice a bit of jitteriness they'll just attribute it to new job nerves so do try hard not to tie yourself into knots worrying what other people think. They probably felt the same way when they were new!
It's easier said than done though, I really suffer from shyness and social awkwardness and tend to replay conversations where I think I've come across weird or silly over and over again, making me more nervous to talk to that person the next time, thus making me come across even more oddly ad infinitum! 
A few things that have helped me are:
-Forget 'just be yourself' (I find this advice so unhelpful!) and make your new mantra 'fake it until you make it', think of how a person who comes across as friendly and confident would act and be that person even if inside you are really nervy (it's not about creating a whole new persona, obviously still be yourself, just a confident version of you!)
- Do plenty of smiling and positive, open body language - standing/sitting up straight rather than hunching, good eye contact. A nice firm hand shake if you normally shake hands etc. A lot of first impressions are based on this rather than what you actually say and it's easier to control!
-Learn some quick breathing exercises you can do at your desk or in the loos or whatever in case of nerves getting the better of you. I do a simple in for a count of 4, hold for 4, out for 4, then in for 6, hold for 6, out for 6 etc one and it calms me down quite well!
-Practice introducing yourself in a mirror or to an understanding family member/friend - just saying the words out loud 'Hello I'm Lilly and I am the new ABC in the XYZ team' (or however you want to introduce yourself) can help you have the words ready when 'under pressure' and not end up in a stupid panicky verbal diorhhea situation or struggling to get any words out at all...
-Take the opportunity of being the new person to ask lots and lots of questions and get people to talk about themselves, explain their jobs etc - people generally like to talk about themselves and it saves you having to say too much! Obviously read their cues about when to stop asking questions though especially about their personal lives- don't pry!
-Have a couple of nice easy small talk type topics ready in case of sudden conversational lulls - the weather, TV, holidays, the state of the roads/public transport etc. I know these are very dull but I like to know I have something 'safe' in my back pocket to bring out in case of awkward silences with colleagues I don't know well whilst making a tea round or in the lift or similar! I figure I'd rather be seen as the boring person who talks about Strictly all the time than the weird silent one and then when I know people better we can have better conversations (in breaks from work of course, don't go the other way and become the office chatterbox)!