I'm scared and I'm frightened are similar. It means you are fearful for some reason. "I'm freaking out" or if you say to someone else, "Don't freak out", usually means overreacting to a situation.

"I'm scared" and "I'm frightened" are interchangeable. I can't think of any difference between them. Both are standard English. "I'm freaking out" is colloquial English. It became common the in 1960s to describe the behavior of people who had taken LSD and were on a "bad trip." It suggests extreme excitement--including fear, but not necessarily fear--together with hallucinations, and bizarre behavior such as yelling and making noise. It suggests a reaction to psychedelic drugs. Nowadays it's used as exaggeration of something milder. For example, someone at work yells a bad word. Heads turn. Someone says "what's wrong with _him?_" "Oh, he's freaking out because they just got another shipment of the wrong parts." "Yes, it's enough to drive anybody crazy."

thanks a lot!

They are all similar in meaning, but there are differences: "I'm scared" can mean you are reluctant to do something: "I'm scared to look at my grade on the final exam" "I'm frightened" always means you think something is going to harm you. "I'm freaking out" is colloquial and means something is making you crazy or uncontrolled. But the most common synonym for scared or frightened is "I'm afraid" (Я боюсь). Just like in Russian, this can either mean you are genuinely fearful of something, or simply that you are apologizing for something: "I'm afraid I can't have lunch with you tomorrow, I have a meeting".