The first thing to notice is that MS Word has built-in styles. You can see them when you are in the "Home" tab. However, Word usually has Calibri or Arial as the default font for "Normal." Although fonts are not specified in APA style, a serif font such as Times is normally recommended. Some teachers can be more fussy, so don't take a chance. Use Times or Times New Roman for your body text.
You can make life even easier by basing one style on another.
The pop-up menu in the bottom right corner (shows "Format" in the image) gives you access to the paragraph settings. That way you can set indents and spacing as well for the style.
Secondary headings in APA format are left aligned bold with upper and lower case.
Third headings in APA style are left aligned bold, with only upper case at the beginning and a period at the end. They are followed by body text on the same line. Because MS Word does not allow you to have different styles in the same paragraph, you can't set a head 3 style. Simple write the paragraph in Body Text style and then select the text and make it bold.
Here's a sample showing the correct headings in APA style.