PostgreSQLs Extension

Hi!
I am developing a work that will need to make some changes in the PostgreSQL source code. More precisely, I'll also make changes in the PostGIS extension. So I work with geographic data.
I wonder if someone can show me a better way for me to make such changes in PostgreSQL:
-> I need to set a new data type specific, as the PostGIS. I saw that in PostgreSQL's Documentation there is a User-Defined Types. This is the best way to define a new data type? Using this approach, can I define the way queries are processed and thus define new operators? Or would I define functions via pgsql for this? It was not clear to me.-> Also, how could I make changes to the SQL language in PostgreSQL?
Sorry about questions, but I would only driving directions, because it is so complex.
Thank you in advance for your attention.

[]sAnderson Carniel