The key is very much where the song feels like it's at rest; at home. It's more likely to show itself in the last chord, which, aftre all, in most songs, is where it finishes, safely back home.
The key you write it in, though, could be any of the 12 majors or 12 minors. If it's sung, that key should be one in which your voice sounds good - not too high or too low, with a little strain if the song warrents it. However, someone else could easily be unhappy in that key, and need to change to suit their voice. Some songs need to be in a particular key for the chords to sound authentic on guitar as a special case, but that's another story.
When writing a song ( I mean 'writing' as in making it up), the key doesn't have to be a paramount consideration. Nor does the time sig. That comes into importance when the song needs to be written DOWN. Sometimes, you might even write a song, and not be aware of its time sig. until then. Doesn't matter. You're too busy being creative!
If it's a song going through your head, it's either major or minor. Establish that first, then play a chord. Any chord, and go from there. When you have the whole thing done, look at the chord pool you've used, and relate that to the key. For example, if there's mostly G, D and A, it'll probably be in G major. G=I, C=IV and D=V, with the G giving the most stable feel to which it will come to rest on.