During Swedish massage, massage therapists use massage oils to facilitate smooth, gliding strokes called effleurage. Other classic Swedish massage moves include kneading, friction, stretching and (sometimes) tapping.
Swedish massage uses firm but gentle pressure to promote relaxation, ease muscle tension and create other health benefits.
Generally during Swedish massage you're nude underneath a towel or sheet. The therapist uncovers only the part of the body he/she is working on, a technique called "draping".
You can keep your underwear on during Swedish massage, and many newcomers do.
Feel free to state your preference for pressure during Swedish massage. It can range from light to firm. Swedish massage usually includes some deeper work on areas of specific muscle tension. If you want more intensive work and firmer pressure, get a deep tissue massage.
As far as exposing your belly, it is up to you, but a professional massage therapists has seen it all, so keep that in mind