I am 12 years-old and I want to write about the girl I wish to marry I want to write the details of her description for me only is this Haraam and if it is a girl of my imagination is it also Haraam?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Islam forbids any means that causes temptation or tribulation to a Muslim, or any means that raises his sexual desires and leads him to adultery or fornication. Regarding the description that you want to write about the woman whom you want to marry, if this is about a specific girl [who really exists] then you are not permitted to do so, and it is not permissible for you to indulge in thinking about her as this is a means of being tempted by her. However, if this 'woman' is indefinite, and you are secure from being tempted by her and the description is free from obscene words, then we do not authoritatively state that it is forbidden but the least that could be said about this is that it is disliked.

Ibn Taymiyyah said: 'It is for this reason that Imaam Ahmad rejected soft love poetry so that one's desires would not be incited to commit adultery or fornication. That is why he ordered persons who were afflicted by passionate love to marry and conceal their passionate love, and thereupon he would be amongst the people about whom Allaah says (which means): {Verily, he who fears Allaah with obedience to Him (by abstaining from sins and evil deeds and by performing righteous good deeds) and is patient, then surely Allaah does not allow to be lost the reward of those who do good.}[Quran 12:90].'

 In the Kuwaiti Fiqh Encyclopaedia it is stated: 'It is permissible to say love poetry about an indefinite woman as long as one does not say obscene words or say something which identifies the person one is speaking about.'

Therefore, in our view, you should totally refrain from thinking about this matter, but there is no harm for a person to endeavour to marry and if this in not possible because of some obstacles, then you should engage yourself in what would benefit you in your religion and worldly matters. Be keen to attend circles of knowledge, keep company with righteous people, and avoid circles of dissoluteness or keeping company with dissolute people.

May Allaah reward you for being keen to know the religious ruling about what you intended to do.  

Allaah Knows best.