I'm currently making a game and want to replace the default fonts. How do I make a font and then replace the AGS fonts with it? Do I need any program besides an image editor like photoshop? Thanks in advance!
To make a completely new font (or edit an exiting one) you need a program like fontforge (http://fontforge.github.io/en-US/).
You can also download a font from the internet and then read up all about fonts and ags game in the wiki article about fonts (http://www.adventuregamestudio.co.uk/wiki/Fonts). Sources for fonts and how to convert truetype to ags compatile format and everything else you need to know is covered there.
Not sure I entirely agree with selmiak's answer. It's all described on the wiki page he links to, but the main point is that AGS supports two different kinds of fonts.
1. Vector fonts in TrueType (.TTF) format. These are the kind of fonts Windows uses. Like Selmiak says, you can create or edit them with a program like FontForge. This tends to be pretty complicated, though.
2. Bitmap fonts in SCI or WFN format (they're pretty much equivalent). These are primitive font formats that aren't really used anywhere else any more, but they're much easier to create. You can simply draw the characters in one of the two font editors for AGS (described on the wiki).
I would recommend the second option. (A bizarre wrinkle in the whole thing is a bug that keeps AGS from importing its own font format, WFN. The workaround is described on the Wiki.)
Finally, as another alternative you can use Calin's SpriteFont plugin (http://www.adventuregamestudio.co.uk/forums/index.php?topic=44359). In this case you'd draw the font in an image editor like Photoshop, and import the font "sprite sheet" as a sprite.