For a shiatsu session you wear comfortable trousers/top and socks, and cloths are used on areas of bare skin. You might lie on your back/sides/front or a combination of all. Shiatsu can also be carried out on someone sitting or standing - but I don't normally do this.
Shiatsu is performed traditionally on a futon or more recently also using a massage bench - I use a massage table.
Before a session it's best to avoid a heavy meal and its also a good idea to allow a stretch of time to be quiet in after a session in order for the experience to sink in.
You might want a one-off session or maybe a few sessions over a couple of months; or occasional sessions more spread out. The aim is to support your health and wellbeing in the way best for you. You might find it goes with focussing and working with other inputs that are beneficial - exercise and other meditation/reflective practices. Shiatsu supports a chance to develop and expand.
The techniques I've learned use relaxed body pressure (acupressure), mainly in the form of palming and thumbing, but also with vibrational, tapping, gentle stretches and rotations and other techniques. Shiatsu connects deeply into the body/mind/emotional centre. This aspect you could see as being a bit like 'Hands-on Reiki'