Im a lvl 40 blood elf warlock where should i lvl?

in an area that has mobs with a level to your liking and quests for your level.

Go to Find -> Quests
and type in the "obtained" fields something like 36-40

I could do it for you and type the result here but frankly all you braindead guys with questions like that can go to hell!

there are lots of places. personally around that level i hung out around ratchet, Stv when I got bit higher closer to 50's range I headed over to UnGoro.

Good areas for a level 40 are still Stranglethorn Vale, I think Desolace has quite a few quests, Dustwallow Marsh has a ton of new quests, Tanaris should be doable, when you get to the mid 40's I'd hit The Hinterlands, after Hinterlands I'd go Un'goro, Felwood, and Winterspring.

That's kind of the order I've gone with two characters so far.  If I ran into a couple of quests that were too hard for my level I'd go back to a previous zone and complete a few "straggler" quests that I couldn't do on my own at that level there.  With my hunter, I've only grouped a handful of times...everything else was solo.

Strangle thorn is a great place for lvl 40, just hit that up then go to Gadget afterwards, and if you have time head to Texas and see my boy Walker for some roundhouse kick power leveling

Yes, SV is a great place then on to Gadget then afterwards head to Texas for some roundhouse training, with trainer Walker.

Coords for Trainer Walker is  18,7 just below the chin

i would say go to strangle thorn then when you hit like 48+ hit outlands maybe by  a mage or so works best then hit like 52 if you happen to spec on demonology then of course get your felguard he can take damage and deal damage hes good for outlands

Outlands at 48? I assume you mean 58? As for the felguard taking damage in Outlands, my first shock in Outlands was just how hard the mobs hit and how fast my FG's health went down. He puts out a ton of dps but does die fast in Outlands without health funnel intervention from time to time.

As for leveling, what I have found to be the best trick is to actually level 2 toons at once. I know this sounds crazy, but for both of my toons I have not leveled without rested XP since level 20. How this works is that I will level one toon until I have reached the rested XP bar, then I switch toons and do the same thing. I also try to stay 1 to 2 levels above the average MOB in an area. The XP you get drops a bit but with the rested XP bonus, the ease of killing, and much less downtime, you fly through the mobs and the XP. The toons do not have to be the same levels so you can just start a new toon now and use my system.

As for where to level, STV has a crap load of quests, then Tanaris, Un'Goro, Silithus, Hinterlands, Western Plaguelands, Eastern Plaguelands...by now you should be able to go to Outlands. I continued my philosophy of being 2 levels higher than the local mobs even into Outlands and it has helped a bunch. I did not go to Outlands until I was 60. I am now at level 69 and not even finished with Nagrand. This allowed me to complete quite a few "group quests" by myself. Sic FG on Mob, DOT, DOT, DOT, Health Funnel, Drain Life, Life Tap, rinse and repeat FTW!