I'm looking for a way to add strings to data(to specific players) e.g:
/comment <player> <comment> and that will save it to storedData or something and then i will be able to do like:
/commentlist <player> and that will show me every comment the certain player has.
Is there a way to do that, i'm looking for a way to do that a long time and i haven't been able to find something.

Check 

Interface.Oxide.DataFileSystem.WriteObject​

I'm not understanding how i can use that in a command, are you able to give me and example?

You need to write a method that parses information you need, retrieves datafile with name you need GetFile("filename") and writes info there (I'd suggest making custom class, but you can use builtin structures (List/Array/Dictionary etc.))
Example:

        [ChatCommand("comment")]
        void CmdComment(BasePlayer player, string command, string[] args)
        {
            if(args.Length > 0)
            {
                var file = Interface.Oxide.DataFileSystem.GetFile("commentDATA");
                Dictionary<string, List<string>> playerComment = 
                file.ReadObject<Dictionary<string, List<string>>>();
                if(!playerComment.ContainsKey(player.displayName)) 
                playerComment.Add(player.displayName, new List<string> { args[0]});
                else playerComment[player.displayName].Add(args[0]);
                file.WriteObject(playerComment);
            }
        }​

I hope you can figure out how to make a response to see all the comments?