Playing the Legendary Edition is definitely the best way to experience the Mass Effect series, especially for a newcomer. It consolidates the trilogy with enhanced graphics and quality-of-life improvements that make the experience smoother. I'd recommend going through the games back-to-back to really appreciate the evolving story and character development, there's something incredibly engaging about seeing your decisions ripple through the narrative.
As for choices, pay special attention to your interactions with characters like Garrus and Tali. Their arcs are beautifully written and have significant impacts on the overall story. Also, don't rush through the side quests; some of them reveal layers about the universe that deepen your understanding of the main plot. Do you have a particular playstyle in mind? Are you leaning towards a certain class or character build?