Without a doubt, you should pay for a couple of lessons with a qualified voice teacher. They'll help you identify your range and point out that with proper training, you'll be able to expand your range both upward and downward.
If you are a man, you should also learn how to take notice of the difference between your "head" voice, your "chest" voice, and your falsetto, and recognize when you are moving from one to the other. Most male singers in pop music make use of falsetto frequently, whether they are aware of it or not, and this has profound implications for figuring out your vocal range.
For better or worse, traditional voice teachers tend not to provide any training regarding falsetto. Somebody who specializes in teaching rock and R&B singers might be willing to work with you on falsetto.
I think I understand what you are saying, but let me point out that Key isn't really the issue. The issue is range, or what in formal terms we call tessatura. That means identifying the lowest note in the melody and the highest note in the melody, which as you can see isn't really dependent on the key of the song, per se. Then you would want to determine whether or not that range of notes sits well in your particular voice.
A good strategy would be to write the entire song first, and then transpose the key up or down until it fits your voice, and then figure out how to play the song in the appropriate key on whatever instrument you play. This is why some guitarists rely on a capo. It's easier to play around with transposition these days because of MIDI sequencers and playback software that can raise or lower the pitch of an audio recording without changing the playback speed.
I'm not a songwriter, but I'm a journalist who has interviewed a lot of them, and one comment I hear from older performers is "I wish I had originally done this song in a lower key, because when I got old, it became really hard to sing it in the key everybody expects to hear it in."