You'll have to complete a raid on the hard difficulty setting, which makes it much more challenging. While this may not be consistent across all raids in the future, in the Vault of Glass, enemies will be tougher, you will not be able to revive fallen allies until you reach the next checkpoint, and certain qualities of the raid are changed. This last bit is hard to pin down and will likely be different from raid to raid. In the VoG, more Oracles spawn during the battle with the Templar, and Atheon spawns detainment shields around anybody he doesn't teleport during the final battle.
This is quite brutal, and is not for the faint of heart. Without a well oiled group working in sync, the raid is nearly impossible on hard. Make sure you go in with friends and be as close to level 30 as you can (tip: you can reach 30 by wearing an exotic helm and normal raid gear in the rest of the slots).
Practice. Practice a lot. Watch videos and read up on the raid you are going to do.
NOTE: Presumably, when the Dark Below launches, the raid will similarly be staggered at levels 28 and 32 (this is speculation), which may allow you to reach a high level 31 or even 32 by completing the normal version of that raid. This can allow you to reach a higher level before returning to do the Vault of Glass raid at level 30. This would almost certainly require that you have the expansion however.