Mario, Marth, Sheik, and Ivysaur are all effective characters that are easy to get a grasp of quickly when starting out. But you really shouldn't limit yourself to these few characters, or any other character's people suggest here - every character in PM is pretty viable at any given level of play. It might take some adjusting to how you should play Olimar - who's playstyle is pretty unique compared to the rest of the cast - but you should be able to pick up just about any character you feel like and get good with them within a few days/weeks or so. Just pick whoever feels natural to you.