The independent t-test is used to test for a difference between two independent groups (like males and females) on the means of a continuous variable.
1) Select Statistics: Compare Means: Independent Samples T-Test (Figure 1 ). A menu like that in Figure 2 should be displayed.
Figure 1: 
Figure 2: 
2) Select continuous variables that you want to test from the list.
3) Click on the arrow that will send them to the "Test Variable(s)" box.
2) Select the categorical variable from which you are going to extract the groups for comparison and send it to the "Grouping Variable" box by pressing the appropriate arrow.
4) Click on the "Define Groups" button. You are confronted with a small dialog box asking you for two groups. In this case, I'm using 1 and 2 (males and females). Click Continue when you're done. Then click OK when you're ready to get the output.
Figure 3: 
5) These are descriptive statistics concerning your variables.
6) This first part is important. You see, there is a possibility for two t-tests to occur here. You have to know which one to use. When comparing groups like this, their variances must be relatively similar for the first t-test to be used. Levene's test checks for this. If the significance for Levene's test is 0.05 or below, then the "Equal Variances Not Assumed" test (the one on the bottom) is used. Otherwise you'll use the "Equal Variances Assumed" test (the one on the top). In this case the significance is 0.287, so we'll be using the "Equal Variances" one.
7) Here's your t statistic.
8) These are the degrees of freedom (df).
9) Here's your significance (two-tailed).