I don't know what composition program you are using, but most of the ones I am familiar with allow you to change the key after you have started working on the music.
The general stereotype is that minor keys sound sadder, while major keys are much happier. This mostly comes from the fact that most sad music just happens to be written in a minor key, and the intervals that make a scale major or minor (third, sixth, and seventh) have more of a sad feel in the minor.
Some composers will tell you that all major keys sound the same, and all minor keys sound the same, so you don't have to worry about which specific key you are writing in as long as you have the major or minor right. I disagree with this - certain keys have certain emotions attached to them specifically (this again comes from the fact that most music with one emotion is written in a certain key). For example, a lot of love songs are commonly written in flat keys (E-flat is a common one), songs that should be more majestic are written in keys like D, G, or B-flat. This isn't always the case - I have definitely written music that fits those generalizations, but have written just as much, if not more, that doesn't follow them (including writing happy music in a minor key and sad in a major). As far as I know, there isn't much reason why this is the case except someone one day deciding to write a piece with a certain mood in a certain key and everyone else thinking that that emotion sounded good in that key and did the same thing.
The three minor scales can be told apart based on their sixth and seventh scale degrees:
Natural minor is the one most people probably think of when they hear the term minor - you take a simple major scale and lower all of the thirds, sixths, and sevenths by a half step.
Harmonic minor is the same as the natural minor, but the seventh is raised because this is the third degree of the dominant chord, which needs to be major for the cadence to be correct.
Melodic minor, which is commonly used in classical music, is the most difficult to tell just from listening to it - going down, it is the natural minor scale, but going up the sixth and seventh degrees are raised.
C natural minor: E-flat, A-flat, B-flat (ascending and descending)
C harmonic minor: E-flat, A-flat, B-natural (ascending and descending)
C melodic minor: E-flat, A-natural, B-natural (ascending); E-flat, A-flat, B-flat (descending).