You can ring up for the for the form beforehand, I believe your initial application has to be done on paper but after that everything can be done by phone. The application can only be filled once babies are here but so long as you get it in within 4 weeks of the birth you will get the money back dated to the birth date.
If you want to know what you will get just google "tax credits calculator" that will tell you exactly what you're entitled to depending how accurate your income figures are, remember they disregard the first £100 per week of SMP/MA.