Before converting to Sikhism, What should a person know about it?

What a great question.
First of all, allow me to congratulate you on your desire to become a Sikh. You are about to make a very good choice achieve eternal satisfaction/happiness in your life.

In preparation for this step. Guru Nanak Sahib mentions on page 1410 of Sri Guru Granth Sahib:
ਜਉ ਤਉ ਪ੍ਰੇਮ ਖੇਲਣ ਕਾ ਚਾਉ ॥
ਸਿਰੁ ਧਰਿ ਤਲੀ ਗਲੀ ਮੇਰੀ ਆਉ ॥
ਇਤੁ ਮਾਰਗਿ ਪੈਰੁ ਧਰੀਜੈ ॥
ਸਿਰੁ ਦੀਜੈ ਕਾਣਿ ਨ ਕੀਜੈ ॥੨੦॥(ਮ:੧, ਪੰ:੧੪੧੦)
The spirit of the message is: If you really wish to indulge in the game of LOVE (of the Creator and the entire creation) please unlearn what you have been taught to-date and accept the message of the Guru. Once you step on this path be ready to sacrifice your previous thoughts without any hesitation,

The essence of this message is that the path of Sikhi is complete faith in the Creator, Its (Creator’s) order (scheme of things) and Love for the entire Creation. The complete emphasis is on way of life ( the intent behind all actions in life), and not show-off rituals.

This is very different than other faiths you might have exposed to previously.

This is all the preparation one needs to have before accepting to be a Sikh.