You use the Royal Exchange to do trades. Each trade gives you a trade ticket and other items depending on which trade you did. When you have 30 trade tickets you can make an expansion permit which is at the end of the list in the Royal Exchange. The option disappears once you've applied them and expanded to the last plot of land that needs a permit. You can hold more than 99 trade tickets.

The inventory guide has more info if you look under T.