First, there are no absolute certainties in life as many things are outside one's control. It is possible to strive towards internal goals, such as being a kinder or more charitable person, but anything external to oneself is ultimately not guaranteed.
One can make meeting goals more probable if one chooses goals realistically. Very few people can manage to become President of the United States or even graduate summa cum laude from an elite university, but completing a two year degree and becoming a manager at a local supermarket or real estate agent are realistic goals for most people in North America.
Another way to increase the chances of completing long term goals is to break them into smaller pieces and focus carefully on each step. Getting a degree involves passing individual courses and to pass a course, you need to succeed one test and paper at a time.
Finally, a key to success is delayed gratification and setting priorities. If a big goal is graduate university, going out to a party the night before a test isn't the way to get there. Although you do want to allow some time for fun, make sure that you aren't missing out on the bigger goals by wasting time on things that don't really matter such as spending lots of time watching TV.