If you put less than 20% down then the mortgage company usually requires that you pay PMI (Private mortgage insurance). This is to protect the company (not you !) in case you default on the loan. PMI is usually very expensive and hard to cancel even after you've paid off 20% of the worth of the house.
So, put at least 20% down. (I always did 21%, just to be sure - because if you're even a dollar less than the 20%, they may still require the PMI.)
The rest of it depends on two factors:
1. What kind of interest are you getting on your investments? For example, if the mortgage is at 7% interest and you are earning 8% on the money you would have used to pay down the mortgage debt, then you'd made more money (1%, in this case) by investing the money and then paying the mortgage every month. (But with interest rates so low right now, you might not make more money that way.)
2. How much debt can you handle ? For one thing, make sure you have enough cash for emergencies (usually about 6 months of your present salary, is the general guidelines, and you keep it somewhere safe - like in a bank CD) and to make any repairs, buy furniture, etc. on the house that you may need. And then, how much are you comfortable paying every month for the mortgage ? If you like to have a low monthly amount, then the more you pay down, the lower the monthly bill is.
Good luck, and congrats on your first home.