How Do I Let Someone Know Im Mad At Them?

You say - "Tim (whoever), you have (really hurt my feelings - betrayed me - lied to me - made me mad when...).  I do not think we can ever be friends again.  Do you have a reason for what you did?"   Listen to what they say and see if it is believable.  Possibly, if you listen with an open mind you will be able to let bygones be bygones.  That does not mean you have to take them back as your good buddy but it does mean that you can get beyond the anger and hurt every time you see the person.  Anger does more harm to you than it does to the other person.  You do not have to trust the other party but you can at least be civil if he has apologized.  Maybe you could accept him as your friend and over time maybe you can learn to trust again???  Only you know how bad the offense was and what it would take to repair the friendship. The answer to your question is simple - speak up.