Many people who are new to list building put their opt-in form on just one tiny area their webpage and wonder why no one is signing up. If your opt-in form is not clearly visible and without priority placement, you're going to get very few sign-ups.
It is a much better idea to make it very visible and above the fold. That means that as soon as someone lands on your webpage they should be able to see your opt-in form without having to scroll down at all. This is especially important for mobile viewers.
You should also make sure that your form is large enough to read and uses contrasting colors from the rest of your site so that people's eyes are drawn to it. Experiment with different priority placements to determine whether the form should be on the left side, right side, or front and center.
Also consider putting your opt-in form at the bottom of your most important posts (think: anchor posts).
Another way to get your opt-in form in front of your potential subscribers is through pop-ups, slide-ins, and “banner” forms. I use Thrive Leads for my opt-in forms.