Colazione is a feminine noun and Italian possessives are always preceded by a definite article (exceptions include members of the family), so the right sentence is:
Questa è la mia colazione.
Remember to put an accent on "è", (it) is, to differentiate it from "e", and.
To have breakfast is translated as fare colazione in Italian, so:
Ora sto facendo colazione.
The -endo verb translates the present continuous.
Your last sentence is missing an article:
Oggi cucino io la cena. You could also say (more common) Oggi preparo io la cena. This implies you have intention of cooking dinner, but you can also say Oggi cucinerò/preparerò io la cena (future tense) to further state your intentions.
Ultimately, it is perfectly fine to day Oggi cucino io.
Hope this helps :)