I love talking to parents about behavior modification and that includes beginning to discipline their children. I really think this is one of the most important jobs for parents and it is hard to believe that your most precious, perfect child will at times misbehave. It happens to all of us!
I would recommend to start using time out as a means of behavior modification when a child is somewhere between 15-18 months of age. For those of you who watch Super Nanny, she coined the word the naughty step which is her version of time-out chair.
When you begin time out, pick a small chair in the house which you can use consistently for time-out. Never use a child's crib or bed, as you do not want them to think that bed is for misbehaving. After a child gets used to doing time out you can use all sorts of chairs and do time out anywhere. Like many things it just takes practice.
When putting your child in time out get down to their eye level, explain why they must sit in the chair, and hold them from behind (with your arms wrapped around them like you are a rope). I use a timer even at this young age so your child begins to understand how long they will be sitting in time out. Time out is typically one minute per year of age.
After time out is finished, get back to eye level and explain that the next time you ask them to mind you, they may choose to listen and they will not have to go to time out. These are such important words for a child's entire life, as they need to understand that they are making choices for their behavior. In other words, taking ownership of making a bad choice and knowing that there will be consequences. You will use these words over and over, you made a bad choice therefore....the consequence is....for a young child it is time out, for older children it may be no TV, or no going to a party, or even no driving. All versions of time-out.
One of my patients is a cute family of 5 and the mother has her version or time-out. She says nose and toes in the corner for a minute----her kids started doing that at 12 months! Impressive.