That's a problem: because without knowing the format of the file, you can't "predict" lines.
Text files for example don't have lines: they have line terminators, which means that in order to access "line n" you have to read the file up to that point, counting line terminators as you go. You can read teh whole file, and teh system will convert it to lines for you:
string[] lines = File.ReadAllLines(pathToLogFile);
But you can't say
string myLine = MyStream.ReadLine(1237);
because liens don;t have to be the same length.
If it isn't a text file, it could have any content: and that make it even worse! I've seen XML, database files, DOCX, XLS, CSV, and loads of proprietary formats used for logs in my time.
So the first thing you need to do is look at your log, and work out exactly what format it has, and then you can start looking at means to identify individual log entries. But we can't do that for you!