The formula you use will depend on whether:

• You have equal numbers of subjects in your groups.
• The standard deviations are equal.
• You are testing the same people twice, using matched groups, or have completely independent people across groups.

The statistic you need to decide whether to reject or fail to reject the Null Hypothesis is the t test. This statistic compares the two means and gets an estimate of the probability that the means are different by chance.