Now that we have seen how much importance Islam has accorded to marriage and marital life you would perhaps ask, “How do we select a spouse? What are the guidelines provided by Islam in this regard? Do we look for some particular characteristics or just try to get the best from the worldly point of view?”
Are Pre-Marital contacts Necessary?
Ali Akber Mazaheri writes:
“The notion that a man and a woman must 'know' each other before they decide to marry, so that they may then be able to live happily together is an illusion. Had there been any element of truth and validity in this, the divorce and separation rates in societies which practice it would not have shown a steady rise. Similarly, the marriages which take place without such pre-marital contacts would not have been known to last happily.”
The Shariah permits the intended spouses to see each other for the purpose of selection and also permits asking and giving opinions if asked (without it being considered as gheebat under certain conditions.)
We should never resort to deceive the opposite party or conceal a defect during the selection process. Such things can have serious ramifications if exposed after marriage
The school of Ahle-Bait (a.s.) has not left us to follow our whims and fancies. We have been taught the best method of selecting a suitable spouse. The most important criterion is piety or religiousness.
Knowing the human weakness for beauty and wealth, the Messenger of Allah (s.a.w.s.) has forewarned, “A man who marries a woman for the sake of her wealth, Allah leaves him in his own condition, and one who marries her (only) for her beauty, will find in her (things) which he dislikes (unpleasing manners) and Allah will gather up all these things for one who marries her for the sake of her faith (religiousness).” 
B. Good Nature
The next important criterion is good nature.
Imam Reza (a.s.) wrote in reply to a person who had asked him if it was advisable to marry his daughter to a person known for his ill nature, “If he is ill-natured (bad tempered), don't marry your daughter to him.” 
The same will apply where the bride-to-be lacks a good nature. Such a woman, though she may be beautiful and rich, would make the life of her husband miserable. She can never be patient in the difficulties that arise in married life.
The Prophet (s.a.) gave no recognition to class distinction, but in marriage, he stressed upon compatibility. The marrying partners must be Kufw of each other, so that there are no unnecessary misgivings later. It is better for a religious woman who is committed to laws and principles to marry a man like herself.
A man questioned the Prophet of Islam (s.a.w.s.), “Whom must we marry?”
He replied, “The suitable (matches).”
“Who are the suitable matches?”
D. Decent Family
The Messenger of Allah (s.a.w.s.) has given great emphasis on taking into consideration a good family background when we intend to marry.
The Holy Prophet (s.a.w.s.) also said, “Look very carefully and minutely as to where you are placing your child because genes and hereditary qualities are transferred in a concealed and unintentional way and have their effect.” 
The Commander of the Faithful, 'Ali (a.s.) strongly forbade marrying a foolish and insane person. “Avoid marrying a stupid woman, since her company is a woe (distress) and her children too get wasted.” 
F. Physical and Mental Health
Though religiousness and piety are most important, it does not mean that we totally disregard the physical appearance and beauty of the prospective spouse.
The Holy Prophet (s.a.w.s.) says, “When one intends to marry a woman, he should ask about her hair, just as he asks about her face (beauty), since the hair is one of the two beauties (of women).” 
G. Whom can you Marry?
“Islamic law has placed certain restrictions on the choice of your spouse depending upon blood relationships and religious affiliations.” Maulana Sayyid Muhammad Rizvi has summarized these laws in a beautiful way:
a. Restrictions based on Relationship
There are certain blood relations which are considered haraam for you as far as marriage is concerned. (As a general rule, anyone who is your mahram is forbidden to you for marriage.) The list of such relatives is given in the Qur'an as follows:
For Man: mother, daughter, paternal aunt, maternal aunt, niece, foster-mother, foster-sister, mother-in-law, step-daughter, daughter-in-law, all married women, sister-in-law (as a 2nd wife) (See the Qur'an, ch. 4, verse 23-24)
For Woman: father, son, paternal uncle, maternal uncle, nephew, foster-mother's husband, foster-brother, father-in-law, stepson, son-in-law.
b. Restrictions based on Religion
A Shi'ah Muslim man can marry: a Shi'ah Muslim woman and a non-Shi'ah Muslim woman. However, if there is danger of being misled, then it is haraam.
He can also marry a Jewish or Christian woman in mut'a only. But he cannot marry a woman of any other faith.
A Shi'ah Muslim woman can marry: a Shi'ah Muslim man or a non-Shi'ah Muslim man, although it is better not to do so; and if there is danger of being misled, then it is haraam. But she cannot marry a non-Muslim man.3
C. Cousin Marriages
Though Shariah does not forbid marriage between first cousins, but there are opinions advocating against them mainly due to a probable risk of the offspring inheriting genetic defects/diseases.
 Youth and Spouse Selection, Ali Akber Mazaheri
 Youth and Spouse Selection, Ali Akbar Mazaheri, Ansariyan Publication
 Wasaelush Shia, Vol. 14, p. 30
 Wasaelush Shia, Vol. 14, p. 31
 Youth and Spouse Selection, Ali Akbar Mazaheri, p. 151
 Youth and Spouse Selection, Ali Akbar Mazaheri, p. 34
 Youth and Spouse Selection, Ali Akbar Mazaheri, p. 175
 Youth and Spouse Selection, Ali Akbar Mazaheri, p. 178
 Makaremul Akhlaq
 Youth and Spouse Selection, Ali Akbar Mazaheri, p. 154
 Wasaelush Shia, Vol. 14, p. 56
 Beharul Anwaar, Vol. 103, p. 237