Sound advice from woofgang there - as usual.
Just because your predeccors got on well with these guys does not make it part of your job description - it's not compulsory to 'join in' with them in any way except what you feel comfortable doing.
You are still very young, and adapting to a workplace takes time for everyone, so don't feel there is anything unusual in your feelings here.
Take it one day at a time, be polite, and friendly, but no more until you have found your feet and feel comfortable around your colleagues.
It will take time for them to know you as well, so just take things steady, and go along with any banter you enjoy, and ignore any that is not what you want to pursue.
As far as a Christmas 'do' goes - again there is no compulsion for you to go - as a new person, you could say you have plans for that day / evening, and they won't know any different.
Your post makes perfect sense - it's all part of the world of work, and the best advice i can give you is to smile, be nice, and only say 'yes' to any invites if you actually want to go. You may feel pressure as the 'new girl' to fit in, but resist it - it's about what makes you comfortable, not fitting into the hierarchy of your workplace.
It does get easier - i started my first long-term job at nineteen, i remember how nervous and ill-at-ease i felt, and how everyone else seemed like close friends with each other except me - it's not usually like that, just how it feels. i am fifty-six now, and still working at the same company, so relax, you'll get used to it!