Play story and do grey bounties. If you have expansion play those stories too, cause you'll get some purple items which are near best items in game. You can start crucible (pvp) at lvl 5 but tbh you'll be miserable, mostly cause you won't have all your abilities unlocked yet. Once you hit level 20 your level will be based off "light points", which come from the purple armor items. If you're lucky enough to find purples as you've been leveling you can easily jump from level 20 to 28 without doing anything. Raids are tricky cause there is no matchmaking so you have to rely on your friends list or 3rd party websites like reddit or destinylfg.com or destinylfg.net which can stink as a noob cause some people won't have the patience to show you the ropes because the levels are incredibly complex and require a lot of teamwork. That being said /r/destinysherpa is a great place for people willing to help you out.