fairly standard question: how can i fight more than one enemFollow

If you want to successfully fight more than one enemy, you will need to make use of your healing abilities. At level 40 as retribution, you should be able to fight at least 3 enemies of your level at the same time without too much difficulty.

You have an ability that heals you instantly using your holy power called Word of Glory, make use of that if you feel you need to. Also, after killing a mob, if you find yourself low on health with another mob attacking you, taking the couple seconds to cast a beefed-up holy light (beefed-up because of the crusader talent) will make a big difference.

Also make use of your Divine Shield or Lay on Hands depending on the situation. (I would use Lay on Hands if the mobs are gradually taking you down, Divine Shield if they are bursting you down via Spellcasting or something)

The thing about fighting multiple mobs, is you want to survive more than you want to kill them. This means that you can't sacrifice all of your survival tools like I mentioned above for dps abilities.

As a side note, Retribution today is not very optimized for killing multiple mobs at once. We have a seal and an ability that will help, but it's nothing like it was during Wrath. It will be more efficient for you to pull one mob at a time, focus on killing it without really having to use your survival abilities, and move on to the next. The guide above is for accidental or unavoidable pulls of more than one mob (murlocs in particular come to mind).

No problem. More often than not you will just learn how to use these abilities and develop your own strategies/tactics for dealing with any situation. It just takes some trial and error.