I'm kind of new to this myself but this is what I've gathered from everything I've heard.
Wet/dry filters are the way to go because they put more water into your system and the more water you have the more stable your aquarium is. Don't get a cheap filter get a nice one because filters are important. If you want a reef aquarium a wet/dry filter is the way to go. You can also put your protein skimmer and other things in the sump so its not hanging off the back of your tank. Canister filters can be nice too but you usually get high nitrates with canister filters and if you have one you need to get a bio wheel unit to get your nitrates down. Undergravel filters don't offer as much filtration.
A protein skimmer is debatable depending on what kind of tank you have. If you want corals and have live rock and have a lot of fish you should definately have one. They're also great for beginners who are trying out saltwater for the first time. You can also put them in your which means they aren't hanging on the back of your tank. Red Sea makes great protein skimmers. Get the best skimmer you can in your price range.
3-5 watts per gallon...you don't want a really cheap one but they're inexpensive. 5-8 watts per gallon if the environment gets really cold in your house or area. Keep your temp between 75-80 degrees but this is debatable.
Keep your water moving...Wavemakers are great if you have corals because you won't have such a constant flow of water you'll have a pulsed flow. You can use 1 or 2 powerheads in your 75 gal
if you don't want corals
You should have live rock in your aquarium because it's natural filtration. It's really expensive, but you can sometimes find good deals online. You should have somewhere in between .75 and 1.25 per gallon but there's no set amount. It should take up 1/3 of the tank. There's also cured and uncured. You can cure your own...there's an article on this site for how to cure rock and liveaquaria.com has a good article about it as well. You can buy cured from your local fish store and put it in your tank but they charge anywhere from 5-15 dollars per pound. You can also buy uncured from your local fish store as well. You can cycle your tank using live rock as well.
Live Sand is great but it's not as necessary as live rock in my opinion. You can use just a normal substrate such as crushed coral. There's a calculator for sand bed depth on this website for how much sand you need. If you can afford it, it is a good idea.
Reverse Osmosis Water
You can buy RO
water which is water that has chemicals filtered out. You can usually buy it at your local fish store and you can buy RO
units, but they're expensive. If you can afford one get one because they get chemicals out of your tap water.
If you want a fish only tank with no corals you can just buy basic flourescent bulbs. If you want corals you want 2-5 watts of lighting for every gallon of water. Metal Halide are the best for reef aquariums and corals. Once again, they're expensive but you can find good deals on ebay and some online sources. I'm not too great at explaining lighting, but I know metal halide is the best for corals. Get a timer to run your lights for 10-12 hours everyday.
Get a good test kit for your pH, ammonia, nitrate, nitrite, calcium, maybe copper. There's master test kits that are great.
Here's a step by step way to set up your aquarium if i were you
1. Get your tank set up and add your substrate. Then add in uncured rock and arrange it where you want. Fill your aquarium with your water and then add salt until the sg
is at 1.025....a good hydrometer can measure this. They're usually under 15 dollars. Install your heater, filter, skimmer, and hood and set your light timer. Wait until your ammonia, nitrite, and nitrate come back down to 0 and cure your rock which usually takes around a month and let your tank run. Do a 25 percent water change once it's your levels of ammonia, nitrate, etc are at 0.
2. Add a cleanup crew with snails and crabs and let those run around in your tank for a couple weeks.
3. Do another 25 percent water change and add corals if you want and wait a couple weeks.
4. 25 percent water change, add fish.
Do a monthly 25 percent water change and make sure you feed your fish once a day and only once a day because overfeeding can kill your fish because the decomposing food will build up nitrates and ammonia (i found this out the hard way). Add certain things to keep your coral healthy someone else will have to tell you about that.
Check with other people about all of this stuff because there's lots of conflicting advice. My best advice is to take it slow and be patient. Don't rush anything. I found that out the hard way because I rushed my tank and it went into disaster mode and i had to start over. Liveaquaria.com has a lot of great articles on it about maintaining an aquarium and so does this website. It also has a compatability chart for fish. Build your tank around the fish you want the most. Put your least agressive fish in first and then build up to your most agressive fish such as triggers. Make sure you build a quarentine tank. You can build one for around 30 dollars. Fill it with your aquarium water and acclimate all your fish to that and keep them in there for a few weeks in case any diseases pop up. This will keep your main tank from getting diseases in it. Also if you have sick fish you can put them in there. a QT tank is REALLY important. My best advice is to do TONS OF RESEARCH. Make sure you get the best equipment you can. Start out with the right stuff and don't rush and do everything the right way and you'll be happy. I hope this helped and if i gave any wrong advice im sorry but that's what i've gathered from most of my research. Hope this helps.