Join the Brotherhood of Steel by progressing their questline
Complete the Brotherhood quest “Eyesight to the Blind”
Receive Power Armor Training from Elder McNamara or Elder Hardin after becoming accepted by the Brotherhood
Alternatively, complete the main quest “For the Republic, Part 2” and gain Power Armor Training from the NCR
Alternatively, complete the main quest “Still in the Dark” and receive Power Armor Training from the Brotherhood
If playing with certain DLC or mods, check for alternate training sources added by them