This is a very mature question, as making ethical decisions will be important. In fact, in my opinion, this is one of the most important decisions in life. In light of this, let me give you a few suggestions.
First, you can read some books on ethics. For example, if you belong to a religious traditions, you can read book on ethics rooted in religion. Most religious have the golden rule - "do unto other as you would have them do unto you." If you are a Christian, there are tons of resources out there. Even if you do not belong to a religious tradition, it is worth delving into this body of literature.
Second, you can talk with others whom you admire. Spending time and getting inspiration from others who live a morally good life is a good way to learn.
Another thing that you can do is to have an "accountability partner." This person can keep you accountable in terms of how you life. Having this other person will give you another conscience.