From what you say, it appears your partner is working now. If so, then you will (as a couple) be getting working tax credit as well as child tax credit, assuming your gross income is low enough for you to be entitled to it. If you work as well, your gross income will be added to your partner's and the result is very lkely to be a reduction in working tax credit - although this will not necessarily occur straight away.
Whether you will be better off depends on how much you earn in relation to how much the working tax credit is reduced. To get an idea of this you need to assume a certain income (in other words, the sort of income you think you would be likely to get) & put all the information into one of the online claculators mentioned. Or go to your local CAB and ask them to do the calculations for you.