It is extremely important to walk on the court for a match with a plan.
A game plan can be as simple as hitting two or three crosscourt groundstrokes and being patient at the baseline. Another simple idea is to serve and volley on all first serves, and work your way in to the net gradually on other points.
Stick with your game plan if it is successful.
Even if you are struggling a little, don’t give up on your game plan until you are significantly into the match (a set and a break down). At that point, sometimes an adjustment can make all the difference, rather than changing your game plan completely.