I want to make a font that links certain letters together when they are typed for example "au" would go together and "ll" would go together. I would also like these two letters together to be underlined when they are typed automatically. Century Gothic font or similar is a fine to use. Other single letters I would like a dot to appear below them. I wonder how I would go about doing this? Any advice welcome. Thanks.
Editing a font is possible in font editor software. There are many of them available commercially and for free. There are a few problems:
others do not have the modified font file, so they cannot print nor watch your documents which use that font (except if you have rasterized or outlined the texts or the font file is embedded) The document will be shown wrongly with a substituted font
fonts are software and someone owns the copyright. Many font copyright owners such as Adobe have strictly forbidden making modified versions of font files and distributing font files even as is. They can have also created technical obstacles for it. Embedding the font inside a PDF is allowed in some cases, when distributing otherwise is strictly forbidden, but that does not help if creating a modified version is forbidden or technically prevented.
Some fonts are free to use, distribute and modify as long as you do it in the way the license states. At least renaming is a must and including all credits of original creators. You should start always with reading the license. If you do not find or understand it, don't make a modified version.
Seemingly you already have got an advice to solve the problem with typography rule settings. Check them first and avoid legal problems.