Know yourself and your abilities. The art of magic should be practiced by those who take it seriously. A trick should never be performed by anyone who does not have mastery of it. Otherwise, tricks will be revealed and you will look foolish.
Write down every trick you know how to perform. No matter the number, list them.
Decide how long you want to perform for. Do you want to go for fifteen minutes? Forty minutes?
Based on the number in Step #3, figure out how long each of your tricks takes to perform. Some will be long, some will be very short.
Choose a variety. Do not do twenty minutes of card tricks. Mix it up so people can see you are skilled in a number of areas.
Plan the scheme and order of the tricks. In a good routine, each trick leads into the next. A great way to start this would be to have a simple "pick a card and I'll find it" trick to begin and then using whatever card you just found, start your next trick.
Start small and work your way up. Nobody wants to see your best trick appear third on a list of ten. Then they know it will only go downhill.