Can Women Work in Islam?
Question

I am a 20-year-old girl studying engineering. I work during the summer in a stationary store; in order to pay my college fees, am I sinful? I wear a Niqab, and sometimes feel that no religious man proposes to me for this reason.
Praise be to Allah.Can women go out in Islam?
The basic principle is that a woman should remain at home, and not go out except for necessary purposes . Allah says (interpretation of the meaning): 
“And stay in your houses, and do not display yourselves like that of the times of ignorance” [Al-Ahzab 33:33]. 
Although this is addressed to the wives of the Prophet (peace and blessings of Allah be upon him), it also applies to the believing women. It is only addressed to the wives of the Prophet (peace and blessings of Allah be upon him) because of their honour and status with the Messenger of Allah (peace and blessings of Allah be upon him), and because they are leading examples for the believing women. 
The Prophet (peace and blessings of Allah be upon him) said: “Woman is `awrah (i.e, she should not display herself in front of men), and if she goes out, the devil raises his hopes (of misguiding her). She is never closer to Allah than when she stays in her house.” (Narrated by Ibn Hibban and Ibn Khuzaymah; classed as authentic by Al-Albani in As-Silsilah As-Shahihah, no. 2688) 
Moreover, he (peace and blessings of Allah be upon him) said concerning a woman’s prayer in the mosque: “Their houses are better for them.” (Narrated by Abu Dawud (567) and classed as authentic by Al-Albani in Sahih Abu Dawud) 
Can women work in Islam?
It is permissible for a woman to go out of her house for work, provided she fulfils certain conditions, such as : 

That she needs to work in order to acquire the money she needs, as in your case.
The job should be suited to the nature of women , such as medicine, nursing , teaching, sewing, and so on.
Her work should not lead to her travelling without a Mahram (her husband or any man whom she is never permitted to marry).
The job should be in a place that is only for women, and there should be no mixing with non-Mahram men .
Whilst at work, she should observe complete Shar`i Hijab.
Her going out to work should not involve committing any prohibited action, such as being alone with the driver, or wearing perfume where non-Mahrams can smell it.
The job should not lead to her neglecting things that are more essential, such as looking after her house, husband and children.

Shaykh Muhammad ibn Salih Al-‘Uthaymin (may Allah have mercy on him) said: 
“The field in which a woman works should be only for women, such as if she works in teaching girls, whether in administration or technical support, or she works at home as a seamstress sewing clothes for women and so on. As for working in fields that are for men, this is not permissible for her because it requires her to mix with men, which is a great source of Fitnah (temptation) and should be avoided. It should be noted that it is proven that the Prophet (peace and blessings of Allah be upon him) said: “I have not left behind any Fitnah that is more harmful to men than women; the Fitnah of the Children of Israel had to do with women.” So the man should keep his family away from places of Fitnah and its causes in all circumstances." (Fatawa Al-Mar`ah Al-Muslimah  2/981) 
If these conditions are met in your work, then there is nothing wrong with you doing it in sha Allah. 
We ask Allah to grant you a righteous husband, for He is able to do that. 
And Allah knows best.