It is great that you are interested in starting a marine tank. It is a hobby that requires much attention. The best advice I can give is to start with as big a tank as you can afford or have room for. Meanwhile do as much reading on the subject as you can. Often ultimately it will be your decision on what to do if you encounter troubles, so arm yourself with knowledge because your aquarium dealer may not always be available or able to give you all the answers to solve your problem.
Many hobbyists start with black mollies to get the tanks nitrifying cycle started.
If there is one biological process the hobbyist should attempt to understand it is this process, the Nitrifying Process. Understanding this cycle will give you much insight into the management of your aquarium. There are many kinds of filtration methods available as well. For the beginner I would suggest an Undergravel filter powered with powerheads and a rear filter as a supplemental filter, kind of like an insurance policy to lessen the effects of an overfeeding. And lastly good lighting is a very essential part of keeping marine fish healthy, Many marine fish depend on good green algae growth to meet their dietary needs.