This was always my favorite thing to teach. The difference makes Spanish so rich.
The Preterit is to show an action that had both a beginning and an end. Imagine a series of photos: he got up / he got washed / he got dressed / he ate breakfast / he left the house. / / / /
Here you have a series of events, each one begun and completed. The English meanings are like: he washed, he DID wash
The Imperfect on the other hand concentrates on the middle of something. Imagine a movie camera grinding away: he was getting up = obviously he began to get up but as far as we know he as never finished that. ~ ~ ~ ~ ~ (to show something was going on. The English meanings would be: he was getting up, he used to get up (repeated action over and over), he got up. The tertiary (third) meaning overlaps with the Preterit and therefore requires more thinking as to whether he DID get up or he WAS gettING up.
In telling a story, the Imperfect paints the scenery or background and the Preterit interrupts with the action.
Feel free to ask questions as you go along. Some verbs have a special meaning in the Preterit or Imperfect and Time is always the Imperfect. This explanation will at least get you started.