When should I use the preterite tense and when should I use the imperfect tense?

The preterite and imperfect tenses are both past tenses, which are used in different situations. 

The preterite tends to describe things that started and finished in the past. The preterite tense indicates completion of an action that ocurred in a certain moment. For example, Esta mañana, me levanté temprano - this morning, I got up early. The preterite tense is used here as we know when the event started and it has been completed. 

On the other hand, the imperfect is used to describe things that happened over a period of time, or several times. The imperfect tells us what something was like or what used to happen. With the imperfect tense, you do not know when the action started or finished but are interested in what happened. For example, Me quejaba mientras me levantaba - I complained whilst I got up. We do not know when this happened, or when it finished and the important piece of information is simply what actually occurred. The imperfect also describes events which happened repeatedly, such as durante el verano, me levantaba temprano todos los días - during the summer, I got up early every day.