Everyone gets the jitters when first faced with codes. Many hospitals have a Mock Code type class which may be helpful. Also, have your ACLS, if you don't already - they cover what you'll need pretty well.
At the end of the day, you won't be in a code all by yourself. When you call the Code Blue (if you are the one who finds the situation), your priority is to BAG THE PATIENT if they are not breathing. The next one in the door can do compressions. Remember your ABC's. Other people are going to rush in, place IVs, someone will grab the code sheet to document. Someone will slap the pads of the defibrillator on the patient, someone will pull open the code cart.. etc. You won't be doing it all alone. The fear comes because it's done so FAST, like everything happening at once. They key is this... if you FIND the situation, and you assess a code situation, YOU call the code and address the situation (Bagging the patient, or whatever) and go from there. If it's your patient, you'll be expected to know the status of the patient (allergies, diagnosis, what's been going on recently, have the chart brought in.
If a code is called and you run to help, take a role. Start out with code sheet if you can. In that - your job is to note the TIME everything happens in order, including vitals. Pick ONE clock to chart by, and pay attention. Code called at X time, Bagging in progress, cpr in progress, med given with the time (the one giving the med will say or shout EPI GOING IN or something similar so you'll know). Things like epi are given usually 3-5 minutes apart so someone might ask "is it time for the next epi?" and your time noted will tell you. You can take all these notes on a blank paper (as long as you do it neatly so you can transcribe properly), then you'll transcribe. Grab a code sheet at work and go over it so you know the info you'll be expected to know.
I was very lucky in my very first code as a new nurse. One of the ICU nurses who responded (a very well known, experienced, published nurse) planted me in front of the code cart and stood behind me and walked me through being the cart nurse, and explained why I should get what ready next. She was awesome and patient and really calmed my anxiety.
Sorry this is so long but I hope it helps. The jitters never really go away, but you WILL NOT be alone in a code situation, so bone up on what you need to know, and breathe!