Well, first off, everyone sounds nasal to themselves when they hear a recording of their own voices. This is because, when you speak, you hear a lot of the internal resonances of the bones in your skull...and you don't hear the effects of your voice in your nasal cavities. Have you listened to anyone else's voice recorded just to see if it's a real problem or just your perception of your own voice?
Your room might be part of the issue but there are a couple of other things you can do. First, there are a lot of singing techniques you can use to "open your throat" and make yourself sound less nasal from the start. I'll let others suggest some exercises--I'm not a vocalist and can't carry a tune in a bucket! However, I've heard the results that a bit of coaching can achieve.
Second, the nasal tone on a voice tends to be from frequencies in the 800-1500 Hz range. A bit of subtle EQ to lower those frequencies a bit can help a lot.
Third, play a bit with mic placement--experiment with the distance between mic and mouth and also try raising the mic a bit the encourage yourself to sing with your head up. You don't want the head tilted way back--just slightly up.
Bob