To capture a legendary follow these steps.
1.First stock up on master balls.
2.Beat all elite four and gym leaders in all regions
3.Search for about 10 - 15 minutes until you find a legendary
Don't listen to the last answer on the 'faster' way to get it. She/he is just trying to hack into your account to set you a new password and email
Chat with our AI personalities
it is legendary
Ionno.
You need to have defeated all the gyms as well as all the E4 before they are released onto the map.
They appear randomly like legendary pokemon's, if u have completed all gyms and elite fours.
It's simpler if you buy a Masterball and have high stat Pokemon.