Getting a haircut?
If one has long hair it is a Mitzvah to cut it [any day prior to Shabbos] in order so one not enter into Shabbos in disgrace. Furthermore, it is a Mitzvah Min Hamuvchar to cut the hair specifically on Erev Shabbos and not beforehand. If however one will not have time to get a haircut on Erev Shabbos then he is to do so on Thursday.
Rosh Chodesh which falls on Erev Shabbos: It is customary not to cut one’s hair on Rosh Chodesh. This applies even when Rosh Chodesh falls on Erev Shabbos. [Thus in such a situation one is to get a haircut on Thursday or Wednesday.]
Washing ones hands after a haircut:
The danger involved in not washing the hands:  If one does not wash his hands after getting a haircut, then he will have inner fear for three days.
How one is to wash: One must use water to clean his hands. It does not suffice to clean his hands in other ways [such as to rub them on something. The reason for this is because an impure spirit resides on ones hands after these actions are done. Thus one needs to wash his hands even if he does not plan to pray or learn Torah afterwards.] However one does not need to pour the water on his hands three times as is required when washing upon awakening. [The entire hand is to be washed until ones wrist, or at least until ones knuckles. The above washing is only in order to remove impurity, and does not prevent one from studying Torah or praying beforehand. ]
May a woman shave body hair other than the head on Erev Shabbos which is also Rosh Chodesh?
Yes. The warning of Rav Yehuda Hachassid only applies to the hair of the head and not to body hair.
When should one have a haircut on Erev Shabbos?
Some rule it is best to get a haircut after midday in order that it is evident that the haircut is being done in the honor of Shabbos. If however one desires to do so beforehand he need not refrain. The Arizal however would always refrain from cutting his hair after midday, and hence would always initially cut it before midday on Erev Shabbos, and so conclude some Poskim. However according to the rulings of Admur in Hilchos Shabbos it is evident that this is not required.
May a gentile give one a haircut? 
Based on Kabala there those which avoid receiving a haircut from a gentile.
Must one wash his hands after cutting someone else’s hair?
 In order so it be evident that one is doing so for the honor of Shabbos. [ibid]
 Due to being involved in the Shabbos preparations. [ibid]
 As one is to proximate the haircut as much as possible to Shabbos in order so it be evident that it is being done for the sake of Shabbos. [ibid] There is no custom against cutting the hair on Thursday, as there is with nails, being that hair grows back the same day as it is cut. [Kuntrus Achron 260/1]
However the Taz [260/1] rules that hair also begins growing back on the 3rd day as do nails and one is to thus avoid cutting them on Thursday. The Kaf Hachayim 260/15 argues on this ruling.
 This custom is based on a warning of Rav Yehuda Hachasid which stated that one is not to take a haircut on Rosh Chodesh due to worry of danger. [ibid]
 Admur writes that the custom to avoid haircuts even on Rosh Chodesh which falls on Erev Shabbos is only followed in certain communities. Today however this is the common widespread custom, and thus no diversion of the custom has been mentioned above.
 Kama 4/18-19
 Magen Avraham and Peri Megadim 4/18. Vetzaruch Iyun as Admur omitted this.
 Kama 4/19; It requires further analysis why washing after a haircut was not listed in 4/18 together with all the other matters listed there.
 Kama 4/19
 Ketzos Hashulchan 2/11. See also Admur 97/3 and Peri Megadim Ashel Avraham 227/2. However see Kaf Hachayim 4/90 which brings an opinion which holds that not all of the following actions bring an impure spirit. Admur however seems to hold that they all do, being that he rules that only washing with water helps.
 Ketzos Hashulchan 2/11
 Ketzos Hashulchan 2/11
 See Admur 7/2; 613/4; 92/6; Peri Megadim 227 A.A. 2 explicitly allows saying a blessing before washing hands from impurity; Mahrsham 4/148 brought in Shearim Hametzuyanim Behalacha 2/5 regarding learning while cutting nails.
In 7/2 Admur rules that one who used the bathroom may learn Torah and say blessings unless he touched his Erva or feces. Now in 4/18 Admur explicitly rules that one who uses the bathroom must wash hands. Hence we clearly see a difference between washing hands from filth and washing form impurity, in which case of the latter it is not an impediment for Torah learning.
The reason: The impurity which comes as a result of the above actions is not the same type of impurity as that which resides when one awakens from sleeping at night, and thus it does not hold the restriction mentioned prior to washing hands in the morning.
 Tzavas Rav Yehudah Hachasid ibid: “A person is not to shave his head or beard on Rosh Chodesh”
 Aruch Hashulchan 260/6; See also 251/3
 M”A 251/5; Kaf Hachayim 260/13
 Kitzur SHU”A 128/15;
The Arizal would not get a haircut past the time of Mincha Gedola [midday], based on Kabalistic reasons. This is brought in Magen Avraham 251/5. Admur however omitted this custom of the Arizal. See Shaareiy Halacha Uminhag 1/130 that the Rebbe concludes regarding the time of cutting nails, which is similar to the time of the cutting of the hair, that he did not receive a directive in how to follow.
Opinion of Gr”a: The Gra is stringent against allowing haircuts past midday, although this is not the worldly custom. [Aruch Hashulchan 251/3]
 Kaf Hachayim 260/13; Mateh Efrayim-Alef Hamagen 581/105
 Kaf Hachayim 4/92
 As it is common for the barber to touch the persons hair which is full of sweat.