Think about how you would do it manually.
You would look at the first character in the string, and compare it with the next. If they are in alphabetical order, you'd add one to a mental count and look at the next pair.
If they weren't, you'd compare the current count with the "longest count" and if it is greater then you have found a longer string, so it becomes the new longest substring.
When you set a new substring, you save the start position as well.
When you get to the end, you know the first longest substring. Try it on paper and you'll see what I mean.
Now look at what you needed to "remember" to do it manually, and think about what variables you will need to hold that, and what the rules are. Now you can start coding to do the same job.
If you are having problems getting started at all, then this may help: How to Write Code to Solve a Problem, A Beginner's Guide