Please go to Walkr > Menu > Mission to view a list of missions and check that you have all the resources you need (no question marks). You can start a mission once you have all the necessary resources.
After starting a mission, you must collect a sufficient amount of the specified resource in order to complete the mission. Different missions will have different rewards. Collecting resources is quite simple -- so long as a mission is in progress, the Walkr system will automatically allocate the required resource to the mission.