new to both taxes and poker

"Burden of proof is on the tax payer." I only paid taxes before once and I don't understand it very well, can anyone clarify. Specifically, if I buy a car next spring the IRS will say "hey where did that money come from?" Is it true that they can just make up some insane income figure and demand payment? If this, or some variation of it, IS possible, how does the "burden of proof" rule apply. If they said I earned 500k and I think i only really made about 50k, how do I prove non-earnings? Unless you give an an explanation of how every hour was spent in a year, how could you ever disprove a bad earning estimate?

If I say "look I made 50k on party." they can say "and live? and pstars? and empire? and dont you cut grass in the summer? dont you baby sit on the weekends? etc" Do they just leave you alone after you report any reasonable ammount? If so, it seems like all OTHER earnings after something reasonable (between 30-100k) is tax free, right?

This year is my first year where I plan to pay taxes as a professional poker player. What consitutes proof of income? I think I remember hearing someone say they just wrote down their net earnings daily and totaled it up at the end of the year. It boggles my mind that you could just write your own record out and they take it on good faith. But if they do accept this sort of thing, I don't see why this wouldn't be enough.

I don't have anything like this and it is not possible for me to find this info. Like most people, I play live sometimes, different sites, different computers, etc. Once I figure out how to, I'm going to do it right next year, but what do I do for now? Would the formula below suffice?

WorkingCapital + cashedout - Deposits = TotalEarnings

Start the year with WC = $0
(Otherwise use the change in WC)
"working capital" = All the money still in your account(s) at years end
"cashouted" = total ammount cashed out in a year
"deposit" = all the money you deposited in a year

What will the IRS do to me if I report this? I'll worry about doing it better in the future.

For simplicity's sake you report no live play.