How do I get to my index.php template?

Hello, I’m new to this and I’m trying to invert the posts on my website so that they’re listed from the oldest at the top to the latest at the bottom. I have instructions on how to do this, but I have no idea how to get to the codex loop, or index, or template files, or wherever it is I have to go to insert the line of code. I could really use some advice. Please help.