Congratulations on reaching this stage in your business plan.
First, I recommend you to follow the steps described by Scott McGregor (few posts above) to get to the initial valuation of what the company can sell for / it's value in the next financing round - call this value post-money ("POST").*
If your partner agrees with the POST value, his ownership share ("SHARE") after he made the investment ("INV") can be calculated as follows:
SHARE = INV / POST
Below are a few things you should consider already at this moment:
1. How much money do you need until the next investment round / next stage of company's development? When will you need another investment (second investment round)?
2. At each round of investment your share will get dilluted. How many investment rounds are you potentially looking at? What stake (and operational control) would you like to keep? Now / at the time of exit?
* In the absence of a business plan and financial projections, this is the best advice I can give about valuation - if you need help with any of those, let me know, I enjoy doing this.