Answer: I came to know from your letter cited above that you do not have a Guru and you are interested in doing Japam and therefore, how to get initiated?
First of all pray sincerely to your Ishta Devata to take you to a right type of Guru who can lead you in the right path towards the goal supreme. Then one day Lord Himself will lead you to the right person who will guide you in the right direction properly and initiate you with suitable Manta. Guru himself will instruct you how to do Japa? In the meantime you can prepare the ground by daily chanting the name of your favorite deity as many times as possible, reading the Holy Scriptures and life & teachings of your Ishta Devata. Cultivate some sweet relationship between you and the deity.