Assuming you have already selected some missions (see: How to Select a Mission) , select one from your journal list and tap on it. Then the mission will begin, you'll start communicating with the person that proposed the mission and be invited to autopilot to the area you need to go to to start the mission. Unless there is a good reason not to do this, Autopilot is usually the best way to travel
Before you begin the mission though you should read the mission requirements to make sure you are the appropriate level to complete the mission for example:
Mission Details
If at any point during the mission you forget what you need to do, simply tap your avatar, then go to the encounters menu and select your current mission from the journal to see what you need to do next.
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