It's probably different for everyone, but one of the things I was most nervous about when starting in ICU was rhythm interpretation-- until the day I disagreed with an intern and I knew
I was right.
He tried to order me to give lido to someone in a-fib, insisting it was V-tach. I was actually still in my critical care residency at the time, not even a full-fledged ICU nurse yet--but I held my ground and said, "I'm not giving lido until you can convince me this isn't afib. Look: narrow complex, irregularly irregular--" He ran off to talk to his resident, then sheepishly came back and asked me to give Digoxin instead. I teased him a little ("Say I was right! Say it!") and he jokingly bowed to me and said he bowed before my superior knowledge. But he never questioned my clinical judgement again after that.
Like any other skill, EKG interpretation gets better with time and lots of practice. Nobody expects you to be an expert right off the bat--and there are lots of books, classes, and tutorials on the web, as well. Walk through each strip logically, breaking it down--measure the P-R, the QRS, the QT--rate, rhythm, ectopics--and if you still don't know what you're looking at, ask a fellow nurse. If the doc has "stumped" you, ask him/her to walk you through the strip and explain what s/he's seeing. Most cardiologists loooove to teach, and they'll respect you for actively trying to learn.
Have fun! There is so much to learn and it's all keeeewwwl!