As everyone here says: Default MS aircraft really suck.
They don't have any form of Flight Management Computer and no Vertical Navigation capability.
As a rule of thumb I don't buy addons for FS (I would rather spend the money on real flying), but then I bought the A320 addon from Phoenix Simulation Software. It rocks. It is commonly accepted amongst flight simmers and real pilots to be the most sophisticated and accurate single aircraft addon currently available. It even comes with an ITVV video on a A320 flight from B'Ham to Zakynthos (spelling?) to try and help you get to grips with some of the functions of the ECAM and EFIS.
http://www.phoenix-simulation.co.uk/
If you get to grips with this then you will feel ready to fly the real thing. It is a great way to get a feel for the aircraft's systems before doing a type rating. I had to chat to a Airbus pilot to understand what I was doing, but what the hey. If I do any flight simming now it is almost always in this.
If you can't be bothered to buy any addon and learn it then look in the FS help files, under the individual help for each type of aircraft it will tell you how to plan a descent in there. As a rule of thumb you are looking at about 3nm per 1000ft.
So if you are cruising at 30,000ft when you are about 90nm out from the airport you should decelerate to 240 kts and commence a descent at about a rate of 1700/1800 ft/min. Monitor the distance and height relationship and adjust the rate of descent accordingly.
Hope this helps,
PT