Seasonal events such as Iron Banner pretty much guarantee 300+ light rewards upon completion, most of the time you're looking at 310+. As for right now, grinding out Court of Oryx can occasionally give drops at 300 light but you best bet really is to do King's Fall. You'd most likely get several pieces of equipment that would immediately take you to 300 light. In order to find a team who will take you, you should use destinylfg.net and look for those doing the normal mode raid from the beginning. That way, you'll learn all the mechanics first hand.