I recently dropped my Canon EOS 600D attached to the 18-55mm IS II lens inside the Canon camera bag from a height of 3ft, and the zoom ring after that is really tight, and I hear a clicking sound when I rotate it, but the auto focus seems to work fine. What can I do now?
The 18-55 is too cheap to repair.
1) use it as is
Replace it with another lens. Warranty coverage includes defects in materials and workmanship, but does not usually extend to user damage.
You can pick up a used EF-S 18-55mm IS II lens at Adorama for about $80-125, or a new one for about $200 on Amazon. It would probably cost more to repair the one you've got. Other options would be lenses such as:
Tamron 17-50mm f/2.8 Di II (non-VC or VC version)
Sigma 17-50mm f/2.8 EX DC OS HSM
Sigma 17-70mm f/2.8-4 DC Macro OS
Canon EF-S 18-135mm f/3.5-5.6 IS
Canon EF-S 15-85mm f/3.5-5.6 IS USM Lens
Canon EF-S 17-85mm f/4-5.6 IS USM Lens
If you can afford it, this is a good chance for you to upgrade to a worthy lens, with bigger aperture.
The cheapest option is Sigma 18-50 F2.8-4.5 HSM OS. $250. Even better is fixed aperture F2.8 for 600$, but as you can see the price jump is substantial.
I have heard good about Tamron's fixed aperture version 17-50mm F2.8 for $500 (not VC, which is significantly worse), but in general I am not impressed by their build quality - I have held more than one Tamron's (owned by friends) that after 1-2 years go "crunch crunch" when you focus it. Never have I seen that with Canon and Sigma, or even 30-60y old vintage lenses.
People worry about non-Canon AF performance, but my Sigmas perform equal or better than my Canons.
Well. There's a few things you can do in cases like these:
Hope that helps!