I am an amateur at this, so I am sure people will come along to correct me
Mountains are mostly formed by moving plates, pushing material upwards or downwards. Material that is pushed upwards forms hills or mountains (depending on their height). So to know where mountains appear, you would need to map all the plates in your world I guess and the way they behave towards each other I have seen some people on this forum make these kind of maps by the way. For me it's too much work. I just pick spots where I would like to see some mountains. Oh, some mountains are formed by volcanoes (the famous volcanic islands in the middle of the ocean!).
As for forests: where they popup depends on many factors. Climate, altitude, rainfaill, soil, etc. Forests can cover immense stretches of land. Humans have been a mayor cause of deforestation though. Most of Western Europe was once covered in forests for example...
It kinda depends on the technology level of your world, but as soon a lot of humans settle down your forests will probably be scattered and there might not be any old growth left. Elves might have a different effect though
(a map of the loss of old growth forest in the US: http://upload.wikimedia.org/wikipedi...Oldgrowth3.jpg)
So if you want to make a totally realistic map you will need to study geology and probably biology. Or at least get an understanding of the basics, which I don't have. It's too complicated for me