As others have said, when you simply start the engine, note the time. When you taxi back in and shut the engine down, note the time again. Yes, if you have a long taxi at your field (maybe busy flight school/corporate hot spot) and then have a 10 minute run up trying to clear a mag and then only simply do one lap in the pattern...it all counts as flight time.
As others have said...wait till you get to the airlines (if thats where you are headed). Id say in excess of 10% of my time* is sitting in line waiting to depart.
*likely much more. It is not uncommon from pushback to takeoff for 30-40 minutes to pass. Figure its a two hour flight and you spent 25% of your logged time taxiing. What brings the average down a little is when you push out of say Syracuse and it only takes 7 minutes from push to takeoff roll.