I'll be succinct here (by my standards at least) - its up to you and your partner to do the legwork here.
for a lot of web links.
Buy Opening Up
and read it with your partner, discuss. If there are still things not settled by that, read some more books.
(on not polyshipping yet - you can probably define that as a very wise suggestion to not get involved with other relationships before having a handle on all the stuff GG suggested above, and really having a clear idea what both you and your partner hope to seek from additional relationships, so other people don't get screwed over or have their hearts broken when it turns out you guys arent on the same page)