Congratulations on starting a saltwater aquarium! First and foremost, I would strongly advise you to keep your guppys in their current 30 gallon tank and buy as large a tank as you can afford and have room for for use as your saltwater aquarium. Size
I say this because the more water you have in your aquarium, the more forgiving your aquarium will be. Waste, etc., will not be as concentrated in a larger body of water, and you will likely have to do water changes less frequently as a result. You will also have more room for more fish, and a greater variety of fish, which means your aquarium may suit your needs longer than a smaller aquarium that you grow to feel isn't big enough. When we first got our 55 gallon aquarium I thought it was huge. Now I would probably purchase a 180 if I was confident it wouldn't crash through the floor of our apartment.
Of course, if your intention is to keep something small like a nano then this logic doesn't apply, but if that's not your explicit intention then a larger tank would facilitate keeping your fish healthy more easily, allow you to have more and a greater variety of fish, and prevent you from tearing it all down and paying for a larger setup a year or two later.
I don't know what your budget and level of patience are, but the fastest way to get up and running is probably to use the pre-cycled saltwater sold in some LFS (local fish stores), "live sand" (which supposedly contains a lot of beneficial bacteria), and fully cured live rock. Otherwise you'll have to wait weeks for the nitrogen cycle (the process of breaking down ammonia into nitrite and then to nitrate) to complete. Sand
As for the sand on the bottom (the "substrate"), the live sand I mentioned is said to have a lot of bacteria beneficial to the nitrogen cycle right out of the bag. From what I understand, you could mix some live sand with certain sand from home improvement stores and it will all become "live sand" eventually. I'm sure you could go with certain sand from home improvement stores by itself and just wait for beneficial bacteria to develop, as I imagine people have been keeping saltwater fish long before bags of "live sand" hit store shelves. Live rock
Live rock is also said to be live because of the bacteria in it that assists with breaking down waste in your water. It is said to be biological filtration. While not strictly
necessary for a fish only saltwater tank, many advise it and some swear by it. We've had no live rock in our 55 gallon tank for the 4 years we've had it until we began adding some very recently, but we've also had consistent nitrate issues that you don't want. I've read that you want anywhere from 1 to 1.50 or 1.75 pounds of live rock per gallon of water in your aquarium, leaning to the higher end if you intend to keep a reef.
It's very important that you don't just get any old live rock and stick it in your tank, though. There is a process called curing, which essentially means killing all of the bad stuff in the rock. You don't want this to happen in
your aquarium. You can either buy fully cured live rock, which will run you around $8.00 a pound based on what I've seen in my research recently, or cure it yourself in a separate tank or tub, but I've heard that curing your own live rock can be a drawn out, messy, smelly process. Protein skimmer
You will also want a device called a protein skimmer. A protein skimmer is a cylinder that forces very small bubbles through an isolated column of your aquarium water. Particles of waste mater in the water ride the bubbles that bubble up to the top of the skimmer where they are removed from your tank. This helps your water quality, of course.
I have to get going, but I'm sure that other members will be able to provide you with additional information about what I've described and make recommendations about filtration, lighting, brands, etc.
I look forward to hearing about your progress!