I first saw Luffy on the cover of issue #2 of Shonen Jump, at a bookstore. I'm not sure what chapters were in there… I think the ones when Zoro first appears. I liked it, and went on buying Shonen Jump for three years. I think I pretty much began collecting manga because of One Piece--although I didn't buy the One Piece manga itself until fairly recently. At the time I would just read it on Shonen Jump.
For a long time, I was too much into Inuyasha to allow myself to dive into One Piece. I know it had the potential to absolutely drain me of time and money, so I decided to wait for Inuyasha to run its course with me, which took years. I still like it, but the time felt right to finally get into One Piece. So I went out and got all the manga that Viz has released so far, and read everything else that has been scanlated. I also watched a couple of the movies and some random episodes of the anime on YouTube (I've steered clear of dubs.) I like the manga most of all, though. I admire Oda-sensei's clean, uncluttered style and how he manages to get so many textures using little to no screen tone. I find it very good for reference when inking my own comics.
Now that I remember, about a year after I first read One Piece I watched one of the movies; I forget which one. I think it was the first one. I do remember not liking it. Ha ha! Luffy's voice just freaked the heck out of me. I found it too weird, I couldn't get used to it. I love it now, though, so I guess the time simply wasn't right back then… (^_^;) His voice is one of the reasons he is my favorite character now.
As to when I really, really began loving it with a passion–that would be when Nami asks him for help, and he gives her his hat before jumping and screaming "Okay!!!" I just loved that, I almost cried.