First of all there is a confusion between number of dependents (exemptions) on the tax return an d the number of allowances on the W-4 form - http://www.irs.gov/pub/irs-pdf/fw4.pdf .
Generally - the IRS expects everyone to pay taxes at the time income is received - that is why tax withholding is implemented. At the tax time - the final calculation should be made and the refund or balance due is determined. Generally - the IRS OKs if the taxpayer owns less than $1000 and pay the balance before April 15. However most people feel uncomfortable if they owe and try to avoid balances.
To estimate tax liability one can use last year return - assuming there will not be major changes - and use the amount from line 63 form 1040 - http://www.irs.gov/pub/irs-pdf/f1040.pdf - This is your total tax.
If there are some changes in financial situation - either fill a tax return - with software - there should not be a problem or use any online estimator - for instance - http://hrblock.com/taxes/tax_calculators/index.html
The next step - to determine the number of allowances for W-4 form - please use the worksheet on the form.
Put 2 if you are single, not dependent on your parent's tax return and that is you primary job
if you itemize add 1 allowance for each $3500 above standard deduction. For instance if your estimated itemized deduction is $14,000
[$14,000 - $5450(standard deduction for singles)] / $3500 = 2
if you have dependent child - add 1
if the child is eligible for child tax credit and your total income will be less than $58,000- add 2
if you have child or dependent care expenses - add 1
So - your total number of allowances is 8
If you find above a little confusing - use Withholding Calculator provided by the IRS - http://www.irs.gov/individuals/article/0,,id=96196,00.html
After you determine W-4 filing - see how you paystub would look like - a payroll person will use tables published by the IRS but we may use this calculator - http://www.paycheckcity.com/NetPayCalc/netpaycalculator.asp
Assuming the person is paid $1000 weekly the paystub will be
Federal Withholding - $54
Social Security - $62
Medicare - $14.50
California - $30
CA SDI - $8
Net Pay - $831.50
Total estimated withholding $54 * 52 weeks = $2808 for the year.
If you compare this amount with your estimated tax liability - that will be estimated balance.