No use asking anybody in Helsinki... see, they don't travel outside Kehä III.
Here is a map of Sipoo:
http://kartta.sipoo.fi/sipoo/map.php?&language=3
Select "Places" from the drop down menu, and write "Sipoon kirkko".
For instance Eniro has also a pretty good route finder:
http://kartat.eniro.fi/
How to get from Helsinki to Nikkilä in Sipoo... this is written from memory, but use the map. You need to get on route nr. 4, which is the road to Lahti, so pick the road going from Helsinki to Lahti. If you for instance drive up Mannerheimintie, you turn right at the Opera House, and then just follow the signs. You should end up driving along route nr. 4, going past the IKEA at Kehä III. Continue driving... soon you should see a sign "Korso, Sipoo", take that exit. Turn right towards Sipoo. Now just drive, and you should end up in Nikkilä. The church should be pretty easy to locate in the center of Nikkilä.