The phrase is "I am interested in..." On a basic level, you simply have to accept that's the way it is. However, "interest" is a transitive verb. What interests you? Philosophy interests me. If I receive the feeling, then I am interested (passive voice). Here, "interested" works as an adjective. To add more information we need a preposition: "in". Philosophy interests me. I am interested in philosophy.

I am interested in will be better because interested in this sentence works as an adjective. However, if you want to use interest you can use it as a verb or or a noun like saying My interests are swimming, dancing and cooking or we have a common interest. Have a nice day.

I am interested in