Levels in this game are determined by the gear you have on, and not the Exp you grind. All the gear you have on now is maxed out, which is why you are stuck at level 24. I'd suggest that you grind some vanguard strikes (the strikes from the playlist on the right hand side of the over world map) until you have over 75 vanguard marks. You can then bring those vanguard marks over to the vanguard mentor and purchase a piece of legendary armor. Your level will go up as that piece of gear is leveled. A few things to note first. You will need to be vanguard rank 2 to buy the gear, so make sure to do any bounties that are available, since that is the quickest way to rank up. You will also need ascendent materials to rank that gear up, which you can get from doing the daily, doing public events, and doing the raid. You likely will not be able to do the raid for a little while though (not until you are more like level 27 or 28).