It may sound like a stupid question but how will I know when I'm ready to flower or its ready to start flowering
You decide when the plant is ready to flower based on how you grow it during the veg stage. Some people will top the plants to get shorter, bushier plants with more colas, this takes more time in veg. The plants below have been vegging since I sprouted the seeds on 4/15, the picture was taken June 30th, they have been topped a couple times. So once they look like this, you can start to flower, or you can keep vegging and grow them even bigger, it all depends on the size of your grow room. You can keep the plant vegging as long as they have more than 12 hours of light per day and the room to grow. Most people will start to flower based on how much room they have to grow.
Also, the plant will stretch taller once it starts to flower, so you have to keep that in mind for the size of your room. If the plant is 2' tall in veg, it can stretch up to another 2' taller once it starts to flower, and that could be too close to your lights.
Since you are growing outside, they will start to flower automatically in August or September unless they have more artificial light from you. Obviously, if they are outside it doesn't matter how tall they get unless you don't want people to see it. It takes experience, once you get over your first plant it will be lots more fun.