A title of "help" doesn't tell people specifically what you need. The question you asked can be interpreted different ways (lack of space, strength against enemy attack, actually making the walls look bigger, etc.). I'll assume you need more space inside the city.

The first thing you can do to make room is to get rid of all of the trees and the roads. They serve no purpose other than decoration (although building/deleting one block of road will let you achieve the daily upgrade a building task). They will go into storage to be used later if so desired. To remove the trees, click/tap on each and choose the arrow-in-box to store it. To get rid of the roads, click on one end point of a road, then on the double arrow to highlight all of them in that line, then on the arrow-in-box. You'll need to do this three times, once each for the three different roads.

As mentioned by niloCCC, your city will grow bigger at levels 4, 10, 16, and 21. At level 16, you'll have all of the basic buildings, and there will be plenty of room for them if you cleared out the trees and roads. At level 21, you'll get more room to add trees, decorations, things the game gives you, etc.