My question is what is the best way of spending the first income of a person? How should he show his gratitude towards Allaah for receiving the job? My second question is that is it allowed for a Muslim to work in banks where the profits come mostly from interest incomes?

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.

The best way for spending one's money is to spend on himself and on his family who are under his custody. After that if he has extra money he may give it in charity for the needy and the poor. Abu Hurayrah narrated that a man came to the Prophet and said: "I have a Deenaar (i.e. golden currency)". He said: "Spend it on yourself." He again said: "I have another one." He said: "Spend it on your children". He again said: "I have another". He said: "Spend it on your wife". He again said: "I have another". He said: "Spend it on your servant". He finally said: "I have another". He replied: "You know best" (i.e. you can spend it in any virtuous field you wish]." [An-Nasaa'ee and Abu Daawood]

Imaam Ahmad narrated that the Prophet said to a man who was possessing eight hundred Deenaars, 'spend it on yourself, if you have leftover, then on your family, if you have extra then on your relatives, if you have still remains then here and there wherever you want (i.e. on virtue).'

As for thanking Allaah for getting a job, whenever one gets good news or is blessed by Allaah he should prostrate to Allaah, which is called, prostration for thankfulness. It is written in Az-Zaad, a book of the Hanbli School 'Prostrating at the time of getting blessings or warding indignation off is desirable.'

Abu Bakrah reported that, when the Prophet received something which pleased him or some glad tidings, he would Prostrate in thanks to Allaah. "Ka'b Ibn Maalik Prostrated when he received the news that Allaah had accepted his repentance." [Ibn Maajah]

The characteristic of this prostration is turning towards the Qiblah in the state of purity, pronouncing Takbeer and then prostrating. During the prostration one says (Subhaana Rabbiyal 'Ala) then raise from it pronouncing Takbeer, then sit and end his prayer with saying Salaam without Tashahhud. Some scholars are of the opinion that this prostration does not need Salaam.

Allaah Knows best.