90 gallons is a good sized tank, might be pricey to get it started up but it will be alot more forgiving than smaller tanks for a beginner.
I would recomend Liverock as the main filtration. You havent mentioned that the tank is drilled so i wont recomend the use of a sump (if you can get it drilled then its definately the way to go with lots more options).
Live rock @ 1kg per 2 gallons is about the minimum you need forit to be able to sustain the tanks filtration on its own.
A good external filter is good for holding spnges, carbon rowaphos etc to help clean the water from small particals.
You havent mentioned what type of tank you want so lets assume for now you want fish only. If thisis so then normal lights will be fine. If you want any form of corals then more powerful lights will be required. T5s are a good option but if the tank is deep (near 24 inches) then Halides are really the best option. If you want Anemones or Hard corals (which i dont advise yet as you are new to the hobby) thenHalides are a must IMO.
Good quality Skimmer, this is even more important with fish only setup. Im afraidi cannot recomend much with skimmers as i dont us eone. The one i use occasionally is a prizm skimmer though.. Good skimmer but there are better out there.
Powerheads. You will need alot of circulation in a marine setup. with 90 gallons you need to be looking for 900 gallons per hour turnover minimum for your tank. (This is advised for corals but i would say the same for fish as they are also used ot swimming in this type or current)
UV filter. Not absolutely essnetial but its a life saver for the beginner. Beginners to the hobby are bound ot make mistakes as they learn more about their tanks (its all part of the learning process). This usually means that fish will be subject to higher stress levels at times etc. This can bring on whitespot which is the bane of marine keepers. There are many effective treamtments but a UV filter definately is another very effective bit of weapnry to your arsenal.
Lastly, get a few good books. ("Reef secrets" is a good book to begin with). research as much as you can about the hobby, its methods, its fish and its corals etc.
Good luck with your erm i mean your fathers setup.
