Ah I see. Well first things first you will have to weed out all the fish that aren't reef-safe.
Some test kits are in order. For calcium and alkalinity I would suggest API or Elos (or get your Salifert's calibrated against more accurate kits as they seem to be fairly unreliable). For magnesium I have found my Salifert to be trusty and easy to use.
If the coral load is low, once you dose the tank to the appropriate level, and pre-dose your water change water to appropriate levels you should be able to keep up to demand simply by doing weekly 10-25% water changes.
For supplements, I have always found that aquarium designed products produce better results (that often only experienced reefers may notice, so maybe not too important for more novice keepers). However, your tank is a significant size and it can get pricey to dose. As far as aquarium designed products go I have used many products and have found Fauna Marin Calcium chloride hydro-somethingorother to be my favourite calcium supplement on the market right now. I haven't tried their alkalinity supplement yet, but i am looking forward to it.
For alkalinity I have been using SeaChem, and it works well. I prefer the Reef Buffer 8.3 to the Reef Builder product as the former mixes easier and helps keep pH over 8.0.
For magnesium I am using littlesilvermax's premixed ratio. I do like it, but I am going to try Fauna Marin's magnesium product when it runs out. I am so impressed with their calcium product I am going to switch all of my "big three" over to their products.
However, for just getting started I really see no reason to not use littlesilvermax's products for all of the "big three". Many seasoned reefers on here use his products exclusively. They are cheap, and they work.