Use a large enough font size in the body of your text so that people can comfortably read it - size 16 is suggested
Your chosen text performs well when it's big or small
Individual letterforms are distinct in shape, making sure that they can't be confused with other letters and are clear/easy to read
Make sure the line height, width and type size are all consistent and clear - lines that are too close together or massively spaced apart can decrease readability by making it harder for the eye to know where to return to when the line breaks
When using headers, try and make sure they are slightly different from the rest of the text by putting them in old for example.
Try and make sure that key information is discernable at a glance.
Design posters/social media posts minimally and intentionally so that the reader can get as much info as quickly as possible. Also consider your required reading order to make sure your information flows and is easy to understand.