You need to pick and set fonts well. Let's cover some simple rules:
Be picky with fonts. Not all fonts are created equal. Compare:
The best fonts tend to cost money. There's a reason for that. However, even with no budget you can do an awful lot with Helvetica or even - ulp - Arial (don't tell any designers I said that). Most of my Quora posts use Helvetica Neue.
Don't mix more than 1-2 different fonts. You can add variety with different weights (e.g. bold, light) of the same font, like this:
Mix too many fonts or sizes and you end up with a ransom note:
Experiment with spacing between letters and lines (tracking and line height). The exact same font can be made to look very different: