For the six months you will earn $21,120. Your tax bracket as a percent of income is 12.85% before deductions and exemptions. The tax on the full amount is $2,714, but you are not taxed on the full $21,120 as you can claim yourself ($3,950 for 2014) so your adjusted gross income will be $17,170. That tax is $2,122. (12.36% of income)
Then you take whatever deductions you have to come up with taxable income.
Since we don’t know if you have any adjustments to income or deductions it is difficult to tell what your tax would be.
Don’t worry about a 401(k), as most companies require six months employment before you even qualify. If you don’t need all your income, open an IRA instead. This will reduce you gross income and lower your taxes.
You need to do the math to determine your taxable income per week and then adjust your W-4 to give you enough withholding. Since you may make changes to the W-4, pick a number to start and then adjust as you work.