Find a list and start studying, simple as that. While doing that, prefer mnemonics over rote memorization (i.e. spend time to think of a story or association to remember each character rather than quickly picking up, forgetting, and relearning the same character 100 times), and prefer regularity over learning speed (i.e. better to look at 10 kanji each day rather than 100 over the weekend; the latter will most definitely fail). Also, learn at least one example word for each (common) reading of the kanji and ideally integrate those into the mnemonic as well.
Most importantly: prevent yourself from looking ahead to the list of characters that you still need to learn, that will only serve to frustrate and discourage you. Only look at the characters that you're going to study that day (and of course the ones you have already studied). Make your kanji study such a strong daily habit that by the time you complete the list, you'll feel confused about stopping
A child can study a new language so fast.
Adult can't. Why?
Maybe we don't have much concentration and studiousness like them!
Children are neurologically pre-wired for language acquisition. As the brain develops, this ability atrophies.
Children have a biological advantage over adults. It has nothing to do with concentration or being studious.