It is obligatory that the Hajji cuts a part of his/her hair of the head or shaves the head and it is his first Hajj then it is better to shave the head, but it is not obligatory. He/she comes out of Ihram by this act, it means that wearing sewed clothes and other Haraam acts of Ihram become permitted for him except fragrance and women that will become Halaal in the next acts (as will be mentioned).
It is an obligatory precaution for those who have tied their hair or have stuck the hair with some materials to shave the head and it is Mustahab precaution to shave the head for those who are performing Hajj for the first time, but it is permitted to only suffice to cutting a part or shortening the hair of the head; and he has choice between these two acts for the next times; although shaving the head is for men and women are not permitted to shave their heads and only cutting a part of the hair of the head is obligatory.
Cutting the nail is good with the cutting of the hair from the head for men and women; but it is obligatory precaution not to suffice to cutting the nail alone.
It is obligatory to perform the triple acts of Mena in sequence. This means that first- Ramy al-Jamarah al-Aqaba, second- sacrifice and third- shaving the head or cutting a part of the hair of the head and nail. Whenever a pilgrim performs these acts without observing the sequence because of ignorance or forgetfulness then his/her acts are correct, even if he/she does not observe that deliberately, he/she has performed a sin but there is no atonement. Delaying shaving the head or cutting the hair of the head or the nail has no problem until the twelfth day, but it is a precaution not to perform these acts in the night.
It is obligatory to perform shaving or shortening of the hair of the head before Tawaf of Hajj, and if a pilgrim performs Tawaf of Hajj before Taqsir deliberately, then he/she should perform Tawaf after Taqsir again and pay the atonement of one sheep; but if he/she has performed Tawaf before Taqsir because of ignorance or forgetfulness then it is has no atonement, but it is obligatory precaution for him/her to perform Tawaf after Taqsir again.
It is not necessary that the Hajji himself/herself shortens his/her hair of the head and it is enough if he/she orders another person to do that, if that person is Shiite or Sunni and in any way he/she himself/herself should make the intention that for example “I perform the obligatory act of Taqsir for Hajjat al-Islam for closeness to Allah (قربة الی الله)”.
It is obligatory to perform shaving or shortening the hair of the head in Mena and if a person does not perform that deliberately or because of ignorance or forgetfulness, it is obligatory for him/her to return to Mena and perform this duty. And if he/she cannot return or it causes severe hardness for him/her, then he/she should perform that anywhere that he/she is; and it is an obligatory precaution to send his/her hair to Mena and it is Mustahab to bury the hair in Mena.
It is not permitted for those who have not performed Taqsir yet and are still Muhrim to shave the head of another person or shorten his/her hair, but they should perform Taqsir first and then they can do it for the others.
A person who knows that if he/she shaves his/her head, he/she might get injured, then it is a precaution to cut a part of the hair first and then shave the head.