wanting to try a duathlon need some guidance

My first suggestion is to check out the free training plans on this site - that's what I did.  I'm not sure if they have a duathlon plan, but you could get one for tri's and skip the swims.  There may be a plan that has a bike focus, that's the one I would use.

Welcome to the world of multisport!

We probably need to know your base that you are starting from.  A few things to consider.

You should be able to do the total milage of running in one shot or very close to it.  This depends on the Du and what the numbers are. 

As far as the bike, you just need to get out and ride.  Also, practice some bricks.  A brick is when you go from one discipline to the next.  In the case of a Du, the hard one is getting off of the bike and then running.  The sensation is differnent for some but most describe having legs that feel very heavy for a 1/2 mile to a mile.  For me, if I put it hard on the bike all the way to the end of the bike leg, my calves will cramp real bad and make me come to a stop.  If I slow down on the last 1/2 to 1 mile on the bike leg, this does not occur nearly as bad.