Reply to Mastering

So I finished the arrangement of my first song in ableton and I feel pretty good about the outcome . I did the track on my headphones mostly as I produce on the go (train, lunch breaks, etc.) and it sounded awesome in the phones, but when I exported the audio out of ableton, it sounds flat and quiet when I play it on my car speakers and even in the same headphones I produced it with. Now I never mastered a track before so what can I do to make it sound like it did when I was working on it in Ableton?

Also any tips on matering for the first time would be very helpful.

Thanks in advance!