The best way to learn/review stereochemistry is to build the molecules themselves until you are comfortable with doing mental 3D rotations. From the diagram it is clearly visible that these molecules are mirror images of one another. Now you need to ask yourself if these two molecules can be superposed. If you were to rotate the second molecule 180 degrees and try to allign it with the first one you would notice that the methyl groups are in different orientations.
Therefore, they are mirror images and they are NOT superposable which means they are enantiomers. I recommend buying a molecular model kit for these types of problems and with enough practice they will become much easier.
