I have worked in several labs before making my decision to apply for graduate school. My proudest moments have been the sensation of accomplishment when finishing a project, publishing a paper, and ultimately recognition for the effort I have put forth.
My first foray into science began in high school. I can still remember the visceral experience from dissecting fetal pigs and getting to see firsthand the inner workings of this fellow living thing. The complexity was fascinating, but the intriguing part to me was the simplicity of how each and every part worked together.
I continued pursuing my interest and worked on several projects in labs while in college as well as after, ultimately publishing in a scientific journal. I think that the experience of having worked on those projects is good preparation for my upcoming project in graduate school. There are many lessons learned in the process, and not just in the academic setting. I felt much more prepared for life in general by learning to work in a team (with fellow scientists), communicating (results, reviews, etc), taking initiative (researching literature for experiment ideas).