where can I find simple Lego ideas
That's a pretty vague question, but here are some general resources on city building:

Rebrickable has a large selection of MOCs. Browse through it and see if you can find something fun there.

You can look through all of the official City sets. Brickset is a good place for that.

Twinlug has a fun Micropolis standard you can use to build a large layout together with other fans.

When you get serious, you might want to get into the modular buildings - it has become a pretty common standard to build for as well.

Hopefully that gives you a start.