depends on what kind of portfolio you like i guess. can be a digital online album or a print. its all your choice. I believe there are many wedding portfolios around here.
a dslr can shoot bw but its always better to shoot in colour then convert to bw as this is a reversible process and you might eventually choose ur colour photo instead. converting to bw via post processing gives you more freeplay with your editing too!~
Other than selecting an on-line album, just start taking photos and put the best ones in your on-line album and wow lah, your portfolio already. Of course, the more photos you have in there the better the credential for your portfolio.
Ya, I agreed that it's better to take the colour photo first then use PS to convert it, cause if you set the camera to take B&W, you can't reverse the process through PS.