I downloaded a font yesterday and it was cursive, so each time i write “T”, well the character appears, each time i write “h” the character appears as well. So if i write Th, the symbol changes into a unique Character forming cursive th. So my question, lets say I write “aum” and i want that each time i write aum(for example), a symbol replace the aum with an special character, another example, if i write in some fonts the diphthong ae, it changes to æ which is an unique symbol, for latin for example.
So I want to write some letters and that a symbol can replace them with an unique symbol, for example if I write “odum” how do i do that the symbol FOR EXAMPLE Ø take its place as if Ø represents “odum”
Thanks 
What you need to do is add OpenType features to your font to make automatic glyph substitutions.
First, you need to create the extra glyphs to be used by the Glyph Substitutions. In the case of æ for latin, it may already exist in the font, and that has a reserved code-point. However, in the case of a special form of Th, that doesn’t have an assigned code-point. Glyphs used by OpenType glyph substitutions don’t have to have a code-point assigned. The OpenType tables can use either postscript names or glyph index (the position of the glyph in the font) to know which glyph to substitute when the user types T then h.
Read the tutorial on Adding OpenType Features
To learn more about using OpenType features, read the tutorial that I wrote for PagePlus. Much of it applies to using OpenType fonts in other applications like Word or InDesign — what are you using?
Making an automatic glyph substitution for aum or Th would look something like this:
lookup ligaSub {
sub a u m -> aum;
sub T h -> Th;
}
The special glyph for aum would have a postscript name of “aum” in this example.
The special glyph for Th would have a postscript name of “Th” in this example.
If you want the ae dipthong to look like a proper ligature, then edit it in FontCreator to join the outlines as in this tutorial: Adding AE and ae to Fonts
This and the change of odum to ødum would be best done with autocorrect in your word-processor.