The first pt I actually worked on was in my hospital rotation during EMT school clinicals. The call came for a code, the rescue was bringing in a pt from a nursing home with a hx of CHF, COPD, diabetes, hypertension, and at the time pneumonia. Being the eager student that I was, I volunteered to jump in wherever I could (there were two other EMT students and a paramedic student there at the same time). When they rolled her in, I realized that she was about 150 kg and I think my jaw literally dropped. That was the first time I did chest compressions on a real person, and I couldn't find any landmarks to go off of. Fortunately the charge nurse helped me correct my positioning a little. The ER doc called her after a couple minutes of CPR and pushing more drugs. After I had dinner, I had to go back to help bag her and transfer her to another gurney for transport to the morgue.
My first call as an EMT was for an infant code, wound up being SIDS. I'm an EMT with the Crisis Response Unit, which is mainly to help provide resources and counseling for family members, so although I didn't work on the pt it was still a tough call. Watching my EMT mentor's partner break the news to the mother that medical control had authorized termination on scene was probably the worst part. However, it was good experience. About two hours into the call (we have to stay on scene until PD leaves, which means detectives, CSIs, and Medical Examiners. This call was 4 hours total) the mother starting complaining of chest pain, dyspnea, and tingling in her fingertips. We figured it was just stressed, but called ALS back just to be safe. I got some practice doing basic EMT stuff, like taking vitals, getting a hx and starting O2 on her. Overall, a tough call emotionally but good for experience.