Not to curse any Jew:[1] 

It is forbidden for a person to express with his mouth any bad tidings regarding any Jew.[2] [One who does so, and mentions Hashems name in a curse against any Jew, irrelevant of language, such as “May G-d do this and this to you,” transgresses a Biblical prohibition and is liable for lashes.[3] This applies even when said not in front of the person.[4] If the curse is said in front of the person, then he also transgresses a Biblical prohibition of Onahs Devarim.[5] If the curse is said without Hashem’s name, then he is not liable for lashes, although it is forbidden to do so.[6]] It is even forbidden to say [regarding a missing person] “if so and so were alive he would have come to here [or would have written a letter, and hence it must be he is dead],” being that a pact is made with the lips [to do their biddings].[7] The same applies with all similar cases one needs to be very careful from what he says. [Even Moshe Rabbeinu, who had good intentions, was erased from Parshas Tetzaveh because he asked G-d to erase him from His book. Thus, certainly a simple person must be careful with his words regarding mundane matters. This is even greater emphasized in the Zohar which states that even a simple person must beware not to ever express any curses from his mouth, as his words cause damage.[8] One who curses another, the curses that he said return to fall upon him.[9] Unfortunately, many people are not careful in this and say G-d will take you etc., and transgress a Biblical prohibition.[10]]

Not to curse oneself:[11] One is not even to curse himself, as there are supernal angels which are called Oreriy Yom which answer Amen, and bring the curse to the supernal snake. [One who does so, and mentions Hashems name in the curse, irrelevant of language, such as “May G-d do this and this to meu,” transgresses a Biblical prohibition and is liable for lashes.[12]]

Not to repeat or recite a curse that can be interpreted to oneself or another:[13] When a person relates to his friend how a certain Gentile [or other person] was cursed, he should not repeat the curse to his friend in that same language, such as to say, “I said to him, you are cursed.” Rather, one is to say, “I said to him that he is cursed.” The same applies when speaking of any negative matter, that one should never mention it in a way that it can refer to him or to another person. [Thus, during conversation, one should avoid saying things like if “such and such were to happen to you” or “if such and such were to happen to me.”]

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^[1] Admur Shemiras Guf Vinefesh Halacha 12; Birkeiy Yosef 116:5; Tziporen Shamir 191; Kitzur SHU”A 33:14; Kaf Hachaim 116:104; Imreiy Yaakov 10:44; Sefer Shemiras Haguf Vihanefesh [Lerner] 225:2

[2] Admur ibid; Kesubos 8b; Chesed Lealafim 116:11

Cursing a Gentile: It is implied from the Poskim that this prohibition does not apply against a Gentile. [See Sefer Chassidim 638; Kaf Hachaim 116:191; Michaber C.M. 27:1]

[3] Michaber C.M. 27:1; Admur Hilchos Onah Ugineivas Daas Halacha 28 [regarding Issur, not Lav]; Rambam Sanhedrin 26:1; Sefer Hamitzvos L.S. 317; Sanhedrin 66a; Vayikra 19:14; Urim Vetumim 27:2;  

[4] Admur ibid; Rambam ibid

[5] Admur ibid

[6] Michaber C.M. 27:2; Kitzur SHU”A 6:3

[7] Admur ibid; Peri Chadash 116:9; See Moed Katan 18a; Sanhedrin 102a

[8] Birkeiy Yosef 116:5; Tziporen Shamir 191; Kaf Hachaim 116:104

[9] Rashi Sanhedrin 49a; Shemiras Hanefesh 228; Kaf Hachaim 116:104; Sefer Shemiras Haguf Vihanefesh [Lerner] 225:4

[10] Tumim C.M. 27:2; Imreiy Yaakov 10:43

[11] Tziporen Shamir 188; Hakdamas Hazohar p. 14 that one who does so is cursed; Kaf Hachaim 116:105-106; Sefer Shemiras Haguf Vihanefesh [Lerner] 225:4

[12] Rama C.M. 27:1; Tur 27

[13] Sefer Chassidim 638 Shemiras Hanefesh 229; Kaf Hachaim 116:191; Sefer Shemiras Haguf Vihanefesh [Lerner] 225:5