Is this interrogative sentence or imperative sentence: "Tell me something about yourself."?

It is an imperative sentence.

The word order (lack of subject) indicates that this sentence is imperative. You ask someone to do something (to tell about yourself).

The interrogative sentence would have order: ##V-S-O##.

An example could be: Could you tell me anything about yourself?

Here the word order and use of anything in place of something indicates an interrogative character of the sentence.