Open up your minecraft game and then click on the 'Profile Editor" tab. Double click on your profile name. This should bring up a box with quite a few little check boxes and such. Next to "version' there is an arrow, clicking on it brings up a little drop down box. Selecting your version is an important step. I recommend using 1.6.4 or 1.7.10 because they have the most developed mods. In this tutorial I will be using 1.7.10, simply because it is a newer version of minecraft.