Hiya,
If you have children already then either or both of you have to work to be eligible for Workign tax credits. If you dont have children I think you have to be over 25 and work more than 30 hours per week. There is an online calculator which will help you determine if you MAY be entitled to tax credits before the baby is born it would be worth doing that.
With regards to the Sure start grant - if you are not on any qualifying benefits or in receipt of CTC then it is unlikely you will get the grant BEFORE the baby is born. visit direct.gov and download the form from there, they will tell you what you need to ensure you can claim the sure start grant. You do have until 3 months after the baby is born to claim it, so it may be better to wait until after the baby is born if you dont meet the criteria now.
At the moment, if you are off sick from work you should be able to claim Employment Support Allowance (ESA) which is the new form of Incapcity Benefit. Again, direct.gov have some information on this. This will involve receiving sick notes from your dr and then sending them into DWP. This is roughly £70 a week and depends on NI credits etc - I think that although you are 17 you should be able to claim ESA as an individual (as your partner works) the other option is JSA, Job seekers allowance, which you could try and claim as a young person (and an individual) , as being pregnant doesnt make you incapable of work until nearer your due date (and only then for 2 weeks post delivery!).
My first port of call for you would be to visit direct.gov and see what help is there for you. AS you are a young person, there may be additional help available. You may also be entitlted to materntiy Allowance if not maternity pay from 29 weeks, and if you are not entitled to maternity allowance then you may get incapcity benifit (ESA).
Good luck! Jill, kayleigh 12, Nico 6 and 32 week very large bump! x