If I sell a stock in Feb. 2014 do I need to file an estimated tax in the first quarter? Does it make a difference as to the amount? Speaking of the capital gains amount.

Depends on the amount of tax the capital gain will generate, and the amount of taxes you are having withheld from your pay check. The basic rule is if withholding accounts for at least 90% of your total tax bill there is no requirement to file estimated taxes each quarter. Also if withholding is equal to at least as much as your total tax bill for 2013 you don't need to so estimated taxes. But even if you meet one of these exceptions, it may still be a good idea to file and pay estimated taxes when you realize the gain, to avoid having to come up with cash to pay off a big tax bill a year from now.