Social Security and Medicare taxes, sometimes called FICA (Federal Insurance Contributions Act) taxes, are automatically deducted from your wages.

Indexed annually, 2007 Social Security withholding is 6.2% of your wages up to $97,500. Medicare withholding is 1.45% of your wages, without limit. 

Why you worried about these two?  They have been automatically deducted from your paycheck (unless you qualify for the exemption).