Which lens do you get first - other than, of course, the kit lens your dSLR probably came with?  It depends on what you’re shooting.
   If you’re shooting weddings, Canon’s 24-70 f/2.8 “L” USM is about essential - as many churches won’t let you use flash inside during the wedding.  It’s been the go-to lens for wedding photographers for some time.  It’s also weather-sealed for outdoor events.  Sure, it’s expensive - but you can make it up on a wedding or two; buy used and version 1 - and save big.
   But for news shooters?  For sports, Canon’s 70-200 f/2.8 “L” IS USM is about essential; it’s also about essential for some news stories.  It’s also expensive, heavy, and large - but substantial savings possible on version 1 and used, and the photos that older version produces look fine printed 20”x30” and will look like the people really are in your living room.
   That leaves the intermediate zoom to buy - and your choices there are Canon’s 24-70 f/2.8 “L” USM, Canon’s 24-105 f/4 “L” IS USM, and Canon’s 28-105 f/3.5-4.5 USM; unfortunately, while all are fine lenses in this wide-to-moderate-telephoto category, only the first two are weather-sealed.  If it comes down to skipping meals out for a while, the 24-70 f/2.8 “L” USM is well worth it.  Otherwise, the 24-105 f/4 “L” IS USM is a fine choice and costs much less - and, if you just can’t afford the others, the 28-105 f/3.5-4.5 is quite good and is quite affordable.  Buying used on any of these - and version 1 on the 24-70 f/2.8 - saves a lot of money for results that very few will notice were any less.  Any will produce photos that - printed 20”x30” - really do look just like the people are in your living room, showing every hair on someone’s head like a movie poster as in the photo of Mitt Romney’s son Tagg campaigning for him (below).
   But - if it came to which one to buy first - I’d buy the intermediate zoom, then save up for that 70-200 f/2.8 on an as-soon-as-possible basis.

Mitt Romney’s son Tagg campaigning for Mitt, Oct. 17, 2012, Burlington, N.C.
24-70 f/2.8 “L” USM lens used.