The best way to introduce a new rabbit is on neutral territory, putting a new rabbit into an existing rabbit's home is almost always going to cause dangerous fighting. A good way of doing this is to take your rabbit with you when you go to choose a new rabbit at the pet shop, breeder, or rabbit rescue centre. If they get along well, then take them both home in the same travel box. This will help the bonding process as they will both be nervous in the car.
When you get them home, again place them on neutral territory it's very important that you don't put them straight into the existing rabbit's home. Supervise them at all times while they are together and assess if they are getting along OK. If they have fallen in love at first sight you will be OK to house them together that night, but make sure there are plenty of places that they can get away from each other if they want to, and provide 2 food bowls etc. More commonly it will take a few careful introductions so make sure you have a spare run for the new rabbit to live in the mean time. House the rabbits within sight and smell of each other and place some of their droppings in each other's run so that they get used to the other smell in their territory. Every day introduce them on neutral territory until you are confident that they will get along, and then you can house them together.
Remember - if you keep males and females together without having them neutered you must be prepared for a sudden increase in the number of rabbits you've got! Breeding is not an area that should be entered into lightly.