I'm not experienced (I've only been practicing and researching for about 7 years and studying folklore and stuff for about 8) but I don't consider myself a complete beginner either so I'll go ahead and throw my experiences in.
My first spell was a summoning spell I found on the internet I believe. I was misinformed and was just doing things for the fun of it, not because of any reason, nor was I considering the consequences of anything I did. (Not saying there's anything inherently wrong with summoning rituals, but I had no idea what I was doing and was not knowledgeable enough to summon.) Luckily nothing bad happened.
I started to practice by researching! After making some questionable decisions in magickat the beginning of my path, I realized how important actually reading was. I spent a long time doing more research than practice, and I think it served well in my practice now, not only by insuring that I knew what I was doing when casting, but also by teaching me researching skills!
I'm not sure if that was what you were looking for so let me know if I misinterpreted something but that's how I started.
I didn't start with spells. I just saw stuff. A forest of masks, a fire, some animal spirits I've befriended. I've also felt energy, so strongly at first that I couldn't stand it. I think my first spell was actually for protection from these energies. It was very effective, but it took a few tries. I kind of regret it though, I can't feel energies so well anymore.
I ended up working my first spell only a year or two after starting to walk a spiritual path. I had ended up coming into direct contact with an entity that had been attached to my older brother for most of his life. He spent years never saying anything about it. The usual 'they'll think I'm just crazy' reasons.
Long story short; I was 16, my brother 19, I had only been reading one book so far involving magic, and the attachment came to light. That night the entity appeared, the rest of the house was asleep save for me, so my brother came and let me know it was present. As soon as I entered his room I felt cold and had the shakes. I operated mostly on instinct, and fell back on the one general spell from the one book I had been reading- spell of manifestation.
The combination of instinct, spell, and "Oh hell no you don't, this is -my- family" proved to be pretty effective as the entity was gone not long after, and suddenly the room did a complete 180 by feeling full of warmth and peace.
Of course I had been calling on help from every source I could think of at the time as well. I think the acting of the spell was just the proverbial pulling of the trigger.
