Fighting with weeds is an age old problem. You may never actually win the war with weeds but you can try to bring them under control. When you start a new garden plot, try to make sure that it is already as weed free as possible.
You want to always get that soil off on the right foot. You might need to spray the plot with Roundup weed killer or some type chemical. Allow it to set for the amount of time it says in the instructions. When the time is up you will be ready to seed.
You want to start off with a seed that is as weed free as it can possibly be. Granted, this seed is going to cost more money but do not get tempted to use a cheaper seed. They are cheaper because they are not weed free. Therefore, they will be in a race with your backyard to see if they can produce more weeds than plant life. In the end, you will work harder and spend more money. That does not sound like a good trade off to me. You will spend more weekends and more money trying to get rid of all those weeds that you could have avoided. Bottom line, use the good seeds from the start, your wallet and your back will appreciate it.