when i start the car up and leave it runing the temp gauge goes up to between 1/4 and haft on the gauge but when i set off the gauge goes down and down,if i stop it will go back up gradually.iv been told it maybe needs a coolant change or a thermostat.any advice wil be great.

Oh by the way......i wanted a subaru for years and im glad iv got one eventually ,it puts a smile on my face!!

 

Change thermostat. and put all new coolant in it.
No point in just changing coolant if the thermostat has gone you will be doing it twice then.
With the weather we been having its always good to change it and keep it fresh. you dont want to end up blowing the engine up with it freezing