Inking isn't something that you absolutely have to do. If the style or tone of your illustration or comic calls for it, or if your penciling is very tight, you can even skip inking entirely. It's all up to you. Personally, I love to ink. For me, it's one of the most relaxing parts of my process and the part that starts to make it look finished. Perhaps that's because my penciling is usually quite rough and messy, and I fill in a lot of details during the inking process.
To start inking, I set my finished sketch layers to a blue line so that I can see the inks that I'm putting down. Then I create an inks layer within the Frame 1 folder, but below any text layers as shown in the following screenshot:
With the inks layer here, it will be affected by the Layer Mask on Frame 1 and so you won't end up with any of your finished lines outside of your panel border.
Now we can take a pen tool and begin to ink the pencil lines. I know that sometimes a lot of artists have trouble with inking, so...