I'm polish, my partner is Indian and he's got resident permit in UK but before he is married to a British and he is not divorced. What nationality my baby is going to have?

First, the child will be polish since you are polish. YOu will probably have to report the birth at the polish consulate
The child will also be Indian through the father. You need to report the birth at the Indian Consulate.
The child may also have the right to the UK citizenship:
http://www.ukba.homeoffice.gov.uk/britishcitizenship/eligibility/children/britishcitizen/borninuk/
Note that not all countries allow multiple citizenships. Call the respective consulates and inquire and make your choices before the baby is born.