Hi Kate, welcome to the forum! I think you have come to the right place for guidance.
First things first, understand that marine filtration has nothing in common with freshwater filtration. You will need to learn filtration concepts from the begging. The nitrogen cycle you learned in freshwater is not helpful in marine environments, because we need to keep Nitrate near zero, requiring a completely different type of filtration.
You will want to use live rock, aragonite sand, and a protein skimmer as your only method of filtration. On a 30 gallon tank you can use a hang on skimmer. Here are a few suggestions: Coralife 65 Skimmer Coralife Super Skimmer 65 AquaC Remora Protein Skimmer with Rio 800 Pump CPR AeroForce Recirculating Protein Skimmer
Each of these skimmers will do the trick, and are progressively better than the prior. The Coralife65 is somewhat maxed out on your 30 gallon tank. You could use the Remora or the CPR AeroForce on a 55 gallon tank if you upgrade later.
In fact, you may want to consider today using your 80 gallon for the marine aquarium. The overall cost won't be much more expensive, and you will have a much much easier task on your hands. In the world of saltwater, bigger is not only easier, it is MUCH easier, especially when we start to predict the comparability of the animals living in the aquarium. Small tanks cramp for space, and often fish and inverts behave in a manor that was not predictable in smaller aquariums.
After you select a skimmer, you will need some live rock. For your project, I suggest an order from 40 Pound box Key Largo Rock, <br>40 Pounds Bahamas Aragonite Sand<BR>pay shipping on rock only - KL40-40
This order of 40 pounds dry rock and 40 pounds of sand will be almost all you need, and is only $109. You can buy about 8 to 10 pounds of live rock to "seed" the dry rock with the necessary bacteria. This rock will create your reef structure and provide all of the beneficial bacteria you need to process nitrogenous compounds, creating Nitrogen Gas as the end product, which leaves the system naturally. This is a huge upgrade from what you have experienced in freshwater, where the end product is Nitrate. More on this here: http://www.tropicalfishkeeping.com/m...shwater-31955/
At this point you will have the filtration taken care of. You will need some other items, such as salt mix, a hydrometer, test kits, calcium supplements, and alkalinity buffers. This part is easy and for the most part you can purchase these after the tank is set up, during the maturing process.
I suggest for research that you visit the Pictures & Videos area of this web site. http://www.tropicalfishkeeping.com/s...ctures-videos/
Here you will find "build" threads that guide you from day 1 on our aquarium set ups. If you read a dozen or so of these builds, you will find successful aquariums that are relatively easy to care for. You will find that all of these tanks have one thing in common... they all utilize the filtration concept described to you above. Whatever you do, do not try to utilize freshwater filtration on a marine tank. This includes every type of biological filter on the market today.
I hope this gets you started well. Feel free to ask questions as you go along.