Two important things
1. Brew another (small) beer first, that is your starter, rack your wort on top of the yeast cake.
2. Ferment such that the wort remains cool. Mid 60's, otherwise you are likly making rocket fuel.
Expect to need to add a growled full of yeast because it is very likely to have a stalled ferment.
A good source is your favorite local brewpub. If not another small beer yeastcake.
Especially if you are not used to brewing big beers,
Go to my website (http://beerdujour.com/Howtobrewabigbeer.htm
) and read how to brew a big beer.