You have to send crusaders on missions. First you need to select one mission on the left by clicking on it (you can scroll down to see all of them). This gives you the details for the mission in the main page (what type of reward, what tags are necessary, how long it would take, the chance of success, and other restrictions for epic mission). Then you have to fill al the circles with crusaders (the ones not in your formation are on the bottom), trying to match all of the necessary tags. This raises the chance of success (100% if you match all the tags). Finally, click the big green button and start the mission. Wait for it to finish, and reap rewards.

As an example, if you wanted to do the mission you selected in your screenshot (farmer brown's tractor) you would need the following tags: DPS, support, female. You can select Jim (DPS and support) and Princess (female) as your crusaders, therefore matching all the tags.