Is she interested?
Jeez?! What in the heck do you think anyone other than her and her closest friends (if she confides with them) would know this answer? Let alone forum members who only know about her what you have shared here? There are literally no mind readers here.
Theres a girl following me insta, she recently text (compliment) about my dp (i dont know her at all) she commented on two of my pics it was like she was totally interested in me, she likes to write to i guess and so do i. A day ago she uploaded a story on insta of a song and to start a convo i text her asking about the name of the song by she didn't reply and only seen the message. What can i do now? How to start convo? How to tell her i like her?
"How to tell her i like her?" Whereas you openly admit that "[you] dont know her at all...."
"she commented on two of my pics it was like she was totally interested in me"
I'm assuming that you've been on the internet long enough to realize that... this really doesn't mean that. She may or may not be interested in you. However you can't (or better yet shouldn't) assume that two mere comments = an intense romantic desire for you. You shouldn't jump to conclusions.
Is this person in your proximity IRL or only online? If the former is an option, you literally start a conversation face to face in order to really get to know her.
If you only know her via online contact? DM her. Talk to her in a chat room. If she's not interested (and was only interested in the above mentioned Instagram posts), she'll let you know. But telling her that you like her when you already admitted to knowing nothing about her? I suspect she's going to eventually block her if you start fawning over her without any establishing back and forth conversation.