FontCreator is a good choice for creating fonts due to its ease of use, but from my limited knowledge of Tibetan you will also need to add OpenType features to render it properly. That can be done with MS VOLT or other free programs, as described in this tutorial thread: Adding OpenType Features.
What OpenType glyph substitution does if the user types “ka” “ka” it will substitute the glyph for “kka.” Only a few programs support OpenType glyph substitutions though, so you need to be aware that your OpenType font won’t work as intended in most Windows programs — if the user types “ka” “ka” that is what he will get. The user would have to select “kka” from a character map to insert it.