Often, you won’t be interested in the questions you ask participants as such, but you are interested in the latent construct that is indicated by a number of variables. In this case you will want to create a scale variable, constructed from your items. To do this you will take the average score of the questions. This means that the scores must all have the same valence: if you score higher on one item, you will score higher on the other. Before you can compute your new scale variable (by calculating the mean of all the items) you should run a factor analysis, and afterwards run a reliability analysis. When your factor analysis shows that your items are one-dimensional and your reliability analysis shows that your items form a reliable scale, you can compute your new scale variable. If all items have positive factor loadings or all items have positive item-scale correlations, all scores have the same valence and you can construct your scale variable. Otherwise, first recode items with negative factor loading or item-scale correlations (see other FAQ entry).
In SPSS, go to ‘Transform’ –> ‘Compute variable’ –> [target variable=scale name] [numeric expression: MEAN(VAR1, VAR2, VAR3, VAR4) ]