Well most of these tanks come with everything you need. IMO there are some essentials you will get when you purchase your tank. And from there on out you will be aquiring things daily or weekly to get you where you want to be.
After you purchase your tank see if it comes with a heater. If not get one I have a 100W for my BioCube 29. Next get your sand, I used live sand and my sand bed isnt as deep as i would have liked it to be, hind site is always 20/20. Anyway I think I have 25 or 30 lbs in there now. You can go home and start it all up with what you have. I recomend using RO/DI water, you can get it at most FS and I know mine sell it salted too. If you are going to buy sea salt and mix your own at least try to get the RO/DI Fresh water and mix it that way. You will need a Hydrometer and eventually a refractometer, I bought the hydrometer for $8.00 first to get me close to the correct salinity. Using SW RO/DI water takes all that guess work and measuring out. A Refracometer is like $40 to $100 depending on which one you get. But when you start adding corals and live fish you'll want one.
After you get your tank running and your sand bed in you can go look for your live rock. IMO I would get all the live rock first, then add fish. Although I didnt, I wish I had. 2 reasons 1st it is a pain to add live rock into a tank that has living things in it, IMO. Moving things around stresses fish and stir's up the tank. Second, it gives the tank time to cycle as you add all your rock in. I dont know about you but it took a good month for me to get all the rock I wanted for my tank I was very selective as most people are.
If you get it all at once live rock and all btw 30 lbs of rock is typical. Set it up in one day and by a pouch of BioSpira you can cycle your tank over night and add fish the next day. Although this method is not what other people will tell you is the right thing to do. I did it with about 10 lbs of live rock and inside of 24 hours I had a Royal Gramma in there, 2 months now shes awesome.
There are tons of things you can do prior to setting your tank up, or after it is running. Wayyyy to much to list, for starters those are the things I decided on and options I chose. You will get tons of recomendations and advise, read and research. I have done hardly none of these mods to date and so far so good. Not saying I wont do them just no need to for me yet. I bought an extra power head to keep the current flowing for no dead spots and I eliminated the carbon filter they give you. Put filter floss on the bio balls and a carbon bag on that in the middle chamber. 15% water change every 2 weeks all is well. Good luck.