I think the most difficult thing to overcome when going from FW to SW systems is that many things you do in FW don't apply anymore or are done very differently. Your almost taking on a new hobby.
Planning in a SW system is everything. There are so many valid choices you can make on equipment, livestock and so on. To get the basic "nuts and bolts" of the hobby down, I recommend you get a couple of good book on state of the art reef systems. Here are two I especially like -
The Conscientious Marine Aquarist by Robert Fenner
The New Marine Aquarium by Michael Paleta <---This book has an especially good section on fish suited to someone starting off in the hobby.
While you can find all this information on the net, there is also a lot of bad information out there and a tremendous volume of information. This can make things very difficult to search through.
When getting books, look for used copies. They will usually be a lot less expensive.
Now the planning part comes in. You need to figure out what you want to keep verses the size of the system you want to build, your budget, and time and effort your willing to devote to it. For example you mention you have kept discus and also arowana. Now you wouldn't recommend those fish to someone new to the hobby because discus have very demanding water conditions and arowana require really large tanks. It's similar with SW.
At this point your saying you want a clownfish. This is not at all unusual, but once you get into the hobby a bit, you'll find lots of other fish, coral and inverts that you may also want. The other livestock may have very different requirements, such as needin a larger tank, better lighting, special foods and so on. Best to plan for this in advance.
Now to look at the specific system you plan.
A 20 gal long tank is on the small side for a SW system. Think of it in terms of a 5 1/2 gal tank for FW. Can you set it up and run it, sure, but it's going to be almost as expensive as a larger tank and be very limited in what you can keep in it. I'd recommend going a bit larger, like a 30 gal tank or a 40 gal breeder tank. The larger volume of water will make life a lot easier for you.
You will also find that you can't crowd SW fish. In FW typically your are told "1 inch of fish per gallon of water", SW generally requires about 5 gal of water per 1 inch of fish. As an example, the 20 gal tank your considering would hold only about 16 gal or so by the time you add live rock and live sand. At best you would want to limit the tank to about two 1 1/2 inch fish, and this would leave little room for growth.
On the filtration, I generally agree with post by
@Humblefish so I need not repeat that. I would also consider getting a skimmer for the tank. In a SW system this can be even more important than the filtration system, since it removed waste peoducts before the biological filtration needs to deal with them.
As for media, a SW system is generally going to contain live rock at about 1 pound of rock per gallon of water. This is the biological filtration system, so you almost never need any bio media in your filtration system. You will generally need some kind of mechanical media in one form or another, and this should be cleaned weekly. You can optionally use various chemical media. Carbon gets mixed reviews in SW, and many opt for other media. Some don't use any chemical media at all. I'd say leave a place for chemical media, start with none at all, and add media you need when you need it.
Lighting is another area in SW systems that is all over the place. If your just keeping fish, you don't need anything special, but if you want corals, you may need some very intense lighting. This is why it's important to plan. You don't want to spend money on basic lighting and then find yourself replacing it soon after because you want to keep those giant clams or SPS corals.
On clownfish and anemones. Like with any fish make sure you start with top quality livestock. Ever since the movie "Finding Nemo" cam out it seems everyone wants a "nemo" and this has cause a lot of low quality clowns in the market. Choose carefully. Anemones that will hose clownfish require excellent water conditions, excellent lighting, and a well established tank, usually one that has been running for about a year with no serious issues. Because of all this don't get a large anemone until you have the experience and system to maintain it. A dead anemone can crash a system overnight, and you don't need that when starting out. If you have to have an anemone stick to the small anemones such as zoas or small rock anemones. They are a lot less demanding.
Lastly, many of your LFSs have little or no knowledge of SW systems. Don't trust them far advice until they prove themselves. Often a store will have only a single person that knows about SW systems. Don't buy anything on a whim, especially livestock, with out checking if it will work out in your system. All to often people bring home something new, and find out it's not going to work.