heather
I would start with looking at the equipment you need as when i put my setup together i thought i had everything i needed until things started to go wrong, then it got expensive all of a sudden.
first off though what sort of marine tank are you loking to set up? fish only, fish only with live rock (FOWLR), or a reef setup with fish/without fish, it is important to think about this now as this will determin the type of lighting you are going to need, having said that if you can afford to i would go with Metal Halide (MH) lighting so if you decide to change your mind at a later date the lighting will be adequate, the choices here basicaly extend to metal halide and t5 lighting, you may want to look into both options at there are draw backs with both.
then you will need to think about filtration, contentous issue this one, as there are a number of different ways this can be done, some people will tell you that if you have live rock you will not need a skimmer, and some will say vice versa, there are primarily two different methods of filtration.
METHOD ONE - A SUMP SETUP
A sump setup is where a smaller is tank housed normally below the main tank where all the filtration takes place via several means including a skimmer in some cases and a mechanical method ie filtration mats much like a normal external, this has a couple of draw backs one is cost and two is space.
METHOD TWO- NON SUMP SETUP
if space is an issue or cost and a sump cannot be used (there is no reason why you cant use this method it really is personal prefference) you can hang the simmer on the side of the tank and use an external filter, couple of draw backs here are one clutter in the tank (pumps and pipes ect) and two the noise of the skimmer.
this part is really down to the individual as to how they want to set this tank up
Lighting is an important decision as if you intend to keep corals the best option would be Metal Halide (MH) lighting as it has a high intensity, couple of issues here are that it
can be expensive to run/buy and it also gives off alot of heat which means it cant be used like a traditional tank where the lid encloses the tank, it haas to be suspended over the top, the other type of lighting is T5 lighting which is adequate for some types of coral and most fish and this can be used in a traditional type of hood as the heat output is not as great as MH, also blue actinic lightin would be required in addition to the marine white lighting.
From here you are going to need power heads to create a flow within the tank (think about the non sump method here as this will be another peice of equipment inside the tank creating yet more clutter)
one peice of kit i would advise if you are going to keep fish is a uv sterilizer as this will help to stop spread of things such as ich or more well known as white spot should a fish develop this, the last thing you want is one fish to get it and it ends up going through the tank! as it could be very expensive.
From here you need to consider which type of substrate to use ie live sand or crushed coral once you have decided this you can finally add water!
but dont forget this has to be RO water mixed with marine salt!.
A few considerations for you
before you add fish you are going to need to test the water aswell as testing the SG (specific Gravity/salinity)with a hydrometer or refractometer so some test kits would be adviseable such as PH, nitrite, nitrate, amonia and KH, also if you are going to add live rock (LR) you will need to know if it has cured as if it hasnt been cured it will cyle in the tank dispersing ammonia and nitrite into the water, this normally takes approx 4 weeks also I found when i set mine up that the substrate was affected slightly.
In addittion to the above i would use a phosphate remover (rowaphos is popular here)in the filter to reduce any algae growth as this is common in new setups and would be inclined to leave the tank to settel for around 4-6weeks with just water and substrate in (and LR if your planning on using some) during which i would add hermit crabs and shrimps after about 3 weeks.
the above is only a guidline for a newbie to marines but i really would suggest visiting ur local fish shop (LFS) and doing a whole load of research into different setups, as for fish compatibility there are a few on here that are very knowledgeable to name a few, skifletch, lynden musho and several more which im sure would be happy to help with the fish side of things
From here on it becomes maintanence which is where the fun begins!
Ok Guys time for you all to tell me what i forgot (if anything.)
musho you got in there while i was writing that one