i have been told that in fs2004 you can do free flights, or whatever you want to call them, and i was wondering wheter you could land at random airports and when you are preparing to land how do you know you will not miss the runway.
You are free to takeoff and land at any airport in the world with MSFS, they won't even stop you from trying to land on a 5000ft runway with your a380...
As to the second part of your question, not sure what you mean or what to say other than you use your instruments and/or look out the front window...
and when you are preparing to land how do you know you will not miss the runway.
Yes, in Flight Simulator you can takeoff and land at all possible airports but before you try a free flight, make sure you've passed some of the basic flying lessons first, as that will teach you how not to miss the runway on landing.
And before you start, you should know that FS is serious business, so don't take it as a game but as a simulation that you really have to work on in order to enjoy it.