If you confirm others' transactions, what do you do to earn 25 bitcoins per block?
Just to give some specifics in addition to David Grayson's answer:

At current difficulty levels, mining a block requires computing an average of


  29,973,900,941,385,007,104


hash operations.  An ordinary computer can perform perhaps 10,000,000 such operations per second.  At that rate, such a computer would take about 95,046 years to mine one block, on average.  If the computer draws 100 watts of electricity, it would use 3,469,201 kilowatt-hours of electricity over that time.  Electrity prices are somewhere around US $0.10 per kWh, so that would cost US $346,920 in electricity alone - much more than the $12,325 of the block reward.

For obvious reasons, most miners today don't use ordinary computers, but rather specialized ASIC hardware that is considerably more efficient (and more expensive) than an ordinary computer.