As Richard says, the 18-55 IS isn't a bad lens at all, tbh, considering it's a bundled lens it's actually pretty good.
The 50mm 1.8 is a cracking lens but as said, the build quality is appalling.
It weighs next-to-nothing, had a ridiculously small focus ring, will come apart with the slightest knock and the noises it makes will drive you mad but............it's a lot of fun and cheap enough to buy even if you rarely use it.
The next step up from the 18-55 is something like the 17-85 USM, it's no winner when it comes to IQ and probably no better than the 18-55 but it has a better range and the USM focus motor is instant and silent.
The 18-135 is supposed to be pretty decent too, still nice and short on the wide end but that bit longer again.
Having said all that, I'm going to agree with what the others have said, get used to what you have before spending money on lenses as you may prefer a certain style of photography.
