2. What's the current specification? Don't look at the stack, look at input and output cases. How well does the code meet the spec? Where is it failing?
3. Before you change anything you need to know what the change process is. You probably do already know this, but if you don't need to find out whether there are any demarcations of responsibility, even if they're only informal and unstated areas of interest.
4. When you have all that, you can start working on the code with some knowledge of the context you - and the code - are operating in.
5. If code works, don't rewrite or refactor for style without a very very good reason. And don't do it unless you can change all the "bad" code at once. Otherwise you'll end up with a mess of incompatible idioms that make future changes hard to read.
6. Write your own docs as you go. Best case is other people will benefit from reading them, worst case is you'll remind yourself what you were doing six months from now - because you'll have forgotten by then.
If you're a junior you may not have access to all of the above, so the fall-back is to find out what you specifically are supposed to do, and where you're supposed to do it.
If that's vague or unspecified, I'd suggest studying the code to make your own model of it and then running possible actions past other team members and the client before you make the first few changes - to establish a working pattern.