Well for a beginner I would go with a 55 gal
tank. if thats out of your budget or size limits then the smallest I would go would be a 30 gal
When at all possible get a tank with the most suface area. Thus if given a choice between a long or high setup go with the long. This will give you more surface area for gas exchange.
Other minimums would be salt mix (obvsiously). A marine test kit for ammonia, nitrite, nitrate, salinity, alkinity at least. If you go with a more natural setup then live rock and live sand for filtration plus a protein skimmer. If your not bothered with that then a good sized canister filter or wet/dry filter and skimmer.
What is important is to know what your goals are to spec a setup to meet those goals. if your only going for a fish only setup then you need less than if your looking for a reef setup.