Start by using the vertical volume sliders on the Reason mixer to balance the relative levels of all your parts. Try not to have any one instrument or sample overpowering everything else, you should be able to focus on each individual element of your song, and hear most of the subtle details without altering any of the faders. The kick drum and bass-line are generally used as a starting point to anchor a mix as you build around them, so set their overall level, and balance each with the other. If you have a lead part, a vocal or melody, set this at a comparable level.
Set the equalization controls --- one for treble, one for bass --- to the neutral "12 o'clock" value Use the "Solo" buttons on the mixer to listen to each part individually, use the equalization controls to accent the particular qualities of each part. If it is a bass part, roll off some of the treble to make it warmer and thicker. If it is a high frequency sound like a hi-hat, remove some of the bass to make it sharp and clean. Subtract rather than add frequencies, as this will keep your final mix sounding clean.
Adjust the pan controls on each channel to position the sounds in the left or right speakers. There is no right or wrong way to this, but it is generally best to leave the kick drum in the center of the mix, as well as any lead melody part that is to be the focus of your song. Any other parts should be spread out, trying to keep roughly an equal amount of sound in both the left and right speaker.
Insert an "M-Class Mastering Suite" from the tools menu. The compressor is the most useful mastering tool you have at your disposal, though it doesn't appear to make the most dramatic difference to the sound. It squashes the peaks of the loudest sounds down and brings the quietest parts up, smoothing the dynamics of the sound, so the whole track can be turned up louder. However, if over-used, compression can make everything sound flat and muffled.
Use the "Threshold" control to set the volume level at which you want the compressor to kick in. Start with the control turned all the way up to "10" and gradually decrease it, listen for the sudden drop off in impact, particularly on percussive sounds, this indicates you've gone too far. The sound will become muffled and flat. Compression is a subtle effect, look at the level meter, for average compression, only 2 to 3 units should be illuminated. The "ratio" control adjusts the amount of compression applied to the sound, whether subtle or severe. The "Gain" control allows you to boost the volume after compression.