#1 mistake people make when first starting out is spending all their money on lights, and never think about ventilation, they think a fan blowing air around is all that is needed,
wrong you turn on a 400w hps in a closet with just a oscilating fan your temps in the winter will be easy 90 + in the summer forget it it will be 500 degrees.
The #1 thing you should set up first is the area were your garden will be, get it all ready, clean it, paint it, etc.
#2 Set up proper ventilation, this will cost you more then the lights, trust me, lights are cheap compared to air conditioners, intake fans, exaust fans, osciliating fans, duck work.
#3 Buy all the tools of the "trade", you will need to know room temp, canopy temp, ph, ec or ppm, if in hydro water temp, etc.
#4 Buy everything you will need soil, pot, fertilizer, if hydro you can build a lot of you trays, reservoirs.
#5 The last thing you buy are the lights, you only need blue light to root, germinate florous work great for that, after that, hps all the way, vegge flower cant beat it, the size of your floor space is irrelevante for chosing a light, what matters is the size of the plant canopy, a 400w is good for 2 or 3 plants tops, or you can do 10 to 12 clones SOG style.
Trust me do it right the first time around, or dont do it at all. It is not as simple or as cheap and many people may lead you to believe, it's expensive to get started, If you can not afford to buy everything all at once buy a little at a time and just stash it in your room for when you are ready.
The bigger the light you plan on buying, the more money you will need to spent on ventilation, and electricity.