The most common concern I hear about from new puppy owners is "biting". So I thought I would take a moment to explain what this biting is and what to do about it.
Biting (often called Mouthing) in puppies under 6 months is a very normal behaviour. When worked on correctly it should disappear by around 4-5 months and sometimes a little later in retriever breeds and terriers. Why do they do it? Well puppies are trying to learn how to use their mouths. Imagine a two year old child and all the grabbing, hitting and pinching they do as they learn to manipulate objects in their hands. It's the same thing.
It's really important for puppies to learn to use their mouths correctly because as adult dogs we can't have mistakes. Our dogs need to know how much pressure to apply, what they are allowed to put their mouth on and what they aren't. This is one of the reasons many trainers don't recommend playing tug of war with puppies (although I personally think it's alright with the right rules in place).
The rule in my house is no teeth on skin but everyone has their own variation of this. When a puppy begins to mouth me and it hurts I let out a high pitch "ouch" and stand up/walk away. This teaches the puppy that by hurting me he makes go away and the behaviour is anti-social. The majority of normal puppies will want your attention and not drive you away.
Another successful tactic is to give your puppy an appropriate chew object. Great toys include stuff animals without stuffing, kongs/hard rubber toys, ropes and chew sticks (antlers, bully sticks, etc). Sometimes you don't have anything handy so instead try asking your puppy for a "sit" or a "hand target/touch" and get them working.
A great way to teach puppies how to use their mouths is by letting them play with other puppies and accepting adult dogs. There's also an argument for keeping puppies with their mother and siblings longer. Having your puppy stay with a good breeder until 10-12 weeks of age can help with mouthing issues. You just want to make sure they're being socialized at the same time. No one wins if the puppy is just locked up in a pen all day.
There is no one size fits all answer and it is something that goes away with age (if you haven't encouraged it). It's important to remember not to hurt your puppy as you don't want to make him/her afraid to have their mouth handled.