For the fastest, most efficient leveling in DDO, a paying subscription is needed. But it is fully possible to level to level 20 on doing F2P content. You are going to have to be smart though, because you will probably run out of quests and content at some point. Leveling a TR (True reincarnation) Character on a F2P account to level 20 will be even harder, if not even impossible. You will also at some point have to run some dungeons with big XP penalties and leveling will get slow. This guide will show you what quests are available to you F2P, you should do all of them on each difficulty at least once. Not only for Xp but for favor.