don't panic, stay calm and positive, you will be fine. you passed interview and got the job didn't you. Keep your head down, learn as much as you can, do the best job you can, try and find a mentor to talk things over with.ignore the tosser who had a word with you, could just be someone sounding off ar trying to get one over you, steer clear from them if you can. Just do a good job on your first few engagements, exceed expectations as much as you can, be proactive and cheerful when under fire, if you make a mistake take it on the chin and move on, ask for feedback from someone you trust, once you have been in a few months you will be right at home.It's unlikely that you have made a bad choice by joining, it's just scary that's all.Worst case scenario is you really don't fit, but you should give it at least 6 months before you reach that conclusion