well, if you are a newbie it would be alot cheaper to start out with orks or space marines, since they come in the starter box, but ill help as best i can.
Start with the assault on black reach box set (85 or 95), this will give you the rulebook (lite version, with only the rules section in it) a few dice + scatter and some templates. you also get some orks and space marines in them, so if you know anyone with those armys you may be able to sell your troop sprues, making the set effectivley free.
after that, you will need to buy the codex book for the tau race (tau empire) which is $33.
after that your gonna need at least 2 troop choices and one HQ to have a game (these are army compulsary choices, so you have to have them) this is where choices are gonna affect your overall cost, so ill use the army i would make if i was a tau player (i play marines) as a sample.
HQ
for a HQ choice you could have either a crisis battlesuit commander ($44) or an ethereal ($18-$20 depending on the model) or a tau named hero which cost 44-55 (i wouldnt reccomend this) .
so we will stick with a standard crisis suit commander for now, since thats the one most tau players would use.
Then you have 2 troop squads, for which you have the choice of kroot or tau fire warriors. ide advise you to have a mix of ranged (tau) and melee (kroot) units in your army, so lets go with a set of each...
fire warriors- $50 for a full squad
kroot- $50 for 16 of them (a full squad is 20)
this leaves $76 for paints and hobby bits.you got half of what you needed for freeish in the black reach box, so now all you will need is paints, glues and brushes. 2 or the 'A J leemans' model brushes (5$ each) will do you for a while. a spray can of basecoat (ide go black) $10. this leaves you with 56 dollars to spend on colours at $5 each.
ill use the T'au sept colour scheme
-skull white
-vomit brown
-scorched brown
-camo green (for kroot skin)
-boltgun metal
plus other colours you may find use of, or are always handy to have
-blood red
-chaos black
-bleached bone
-brazen brass/ tin bitz
then all you need is some sand (15) glue (5) and a little flock (7ish) to round out the bigginers army.
its also possible tp pick up models 2nd hand on ebay and the like, however you might find them already painted, even varnished, and assembled. i know myself i hate painting models already built, as i paint them by peice before assembley, and going over the prior paint job will obscure fine details as it fills up with paint.