Marrying into a family is tough and I can't say much on that subject directly.  But trying to reign in a child that has been raised by someone else is something I have done.  The first thing I can think of is to get the Bio-Parent and yourself on the same page.  Discuss goals and expectations thoroughly before doing anything with the child.  Most important is to decide what role you will have in diciplining the child.  An older child will simply not accept you as "Mom" or "Dad" but you may still end up with the role of diciplinarian.

Some things that I find crucial to taming an out of control child:

1. Clearly defined rules.  And don't go overboard.  Pick some basics and stick to them.  Write them down on a chaulk board in the kitchen.
2. Clearly defined schedule.  Kinda like #1.  This is so important to making a child feel secure.
3. Clearly defined punishments.  The same infraction gets punished the same way every time.  Like a formula.  But besure that deliberate disobedience is punished at the highest level or the child will go "punishment shopping", that is doing things on purpose knowing that he will get punished lightly.  Your word must be obeyed every time.  Only issue commands when it is really necesary.
4. Use physical punisment ONLY when a non-corporal one will not convey the seriousness of the punishment.  I may be in the minority here but a hand to the diaper is not abuse and is often the only way to impress upon a 2 year old the seriousness of running away in a parking lot.  That said at 3 or so a child should have the cognative ability to understand a scolding, time-out, etc.
5. Reward every good behaviour with praise.  It should be a rare treat to be rewarded with "stuff"  (like potty training maybe).  While you should pick and choose only the most necessary infractions to punish, reward every good behaviour.

Okay, that should get you started.  The key is consistancy.  Time-out does work (made a believer out of me) but it must be enforced every time.  Each trip to time out should end with an explanation of why the child is sent and the child needs to apologise for it.  Then a hug and an "I love you" and send them off to be kids again.  No grudges.  The punishment for that crime is done.

Good luck.