Buzzy, you say you have a 450 liter tank with nothing in it. You mean nothing as in totally empty, no water, nothing?
The reason I ask is if you are totally new to marine tanks, there is much to know before even starting, more than can be explained in one post. The best plan would be to meet up with someone who is running a successful tank and discuss with them.
But for you, the basics would be this :-
The 450 liter tank is big enough for a smallish snapper. For filtration you will need liverock and circulation. External filters are not such a good idea for marine aquaria. Liverock is made from coral skeleton, and houses bacteria that eat the waste in the tank. The normal amount of liverock is about 1 kg to 6 liters water, so for your tank 75 kg. You can have less, depending on the bioload. For the rock to work effectively, you need sufficient water movement. This is achieved by pumps or powerheads, the flow should be at minimum 10 times tank volume per hour, or for your tank, you would have pumps with a total of at least 4,500 liters per hour.
When you initially put the rock in the water, it has to be "cured". In other words, the rock will contain dead organic matter that will pollute the water, and has to be given a few weeks to break down before fish can be added.
Get this right, and you have a basic setup that could house a snapper. The skimmer can come later, lighting is not critical in a fish only set up, and don't mess with any external filters.
But there is MUCH to learn. Here is an excellent site with a wealth of info. http://reefcentral.com/