Tax Credits are an income related benefit, dressed up as something else. There's 2 types, working and child tax credit. you apply for both at the same time and they can also cover childcare costs. higher earners will likely only get child tc.
You should read the notes that come with the form, to see how maternity pay is treated - part of this is disregarded. sorry can't remembr off the top of my head.
as said above, they are initially based on your income for the previous year, however, you can call them and correct this if your household income has changed this year. someone up there has said don't apply because they are a nightmare. it's true that they can cause many problems, mainly via their poor administration, and then responsibility for the error is usually passed onto you. They way to avoid this is to be careful and record the date and time when you call them and the name of the person you speak to.
you have to make sure you do respond within their time limits if they ask for more information as well
both tax credits and child benefit can be backdated 3 months automatically (assuming you're entitled)
there's also something new called a 'health in pregnancy' grant of err £190 i think. googling should find details for how to apply
i'm a benefits adviser, so feel free to email me if you have any specific questions. i'll give you a better answer than most on here