Its an awful idea actually. Many people pick up bad habits that then have to be broken by instructors. You aren't going to do yourself any favors by messing around on flight sim before you take lessons.
Now, once you start taking lessons its a different story. flight sim is great for practicing things like steep turns, stalls, slow flight, radio navigation, and its especially helpful with emergencies. Basically anything except ground reference maneuvers and landings. Its a great idea to go to the airport, fly your lesson then come home and fly it over on your sim, working on things you had a problem with as long as you get the idea. Its also great with helping you with flows and checklists.
Once you learn how to fly its good for keeping you somewhat up to speed. When a pilot takes time away from flying, we forget things. usually its not big things but it can be. It all depends on the person, the time taken off, and the complexity of the airplane.
The best part of flightsim is practicing instrument flying. It really does work great for that, but again you need to take lessons from an instructor before you go off teaching yourself how to fly on instruments.
Its a great tool, but you need to learn how to use the tool first.