Which Camera Lens?

After consulting with people in the know I bought a new camera recently.
It’s a Canon EOS 400D with a EF-S 18-55 mm Lens.
You can see some of the photos taken with it during my trip to Lisbon.
The problem I now face is lens.
Which ones do I need?
Where can I get them?
How much should I spend on them?
I am lost as usual!