You'll want to use headings to structure your lit review. The APA 7th Additions specifies five levels of headings, in descending levels of emphasis (i. e. level 1 headings are the most important and level 5 headings the least). The number of headings you use will depend on the length and complexity of your paper, but in any case, make sure to begin with a level 1 heading and proceed sequentially to level 2, then level 3, etc.
The introduction to your paper does not begin with a heading of any kind. Note that there cannot be single level 3, 4, or 5 headings. That is, you must have more than one heading at each of those levels.
Level 1 Headings Are Centered, Title Case and Bold With No Closing Period
Start a new, indented paragraph on the next line after Level 1 headings.
Level 2 Headings Are Flush Left (Not Centered and Not Indented) and Are Title Case and Bold With No Closing Period
Start a new, indented paragraph on the next line after Level 2 headings.
Level 3 Headings Are Flush Left (Not Centered and Not Indented) and Are Title Case, Bold, and Italicized, With No Closing Period
Start a new, indented paragraph on the next line after Level 3 headings.
Level 4 Headings are Indented Left, Title Case and Bold With a Closing Period. Continue writing the paragraph on the same line after Level 4 headings.
Level 5 Headings are Indented Left, Title Case, Bold, and Italicized With a Closing Period. Continue writing the paragraph on the same line after Level 5 headings.
