I can`t afford to get married, what should I do to curb my sexual drives?

You should offer a lot of voluntary fasting, keep busy with useful and permissible acts and make supplication to Allah, The Exalted.

Is it permissible to offer mandatory prayer while sitting?

All perfect praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions.                                                                                                                                                                                                                            Standing up is a pillar in the mandatory prayer and the prayer is not valid without it unless one cannot do it. As for Nafila (supererogatory prayer), one can perform it in the state of sitting even if he/she can stand, but his/her reward is half the reward of the person who prays while standing if he/sh has no legal excuse for such an act. And Allah Knows Best.

Is Zakah (obligatory charity) due on leased property, and how is it calculated?

Zakah isn`t due on the leased property itself, but on the saved income from that property if it reached Nissab (minimum amount liable for Zakah), and a whole lunar year had lapsed over saving it.

I have 490 shares in a commercial company, and they have been held for a year. The price of each share is one dinar, noting that the shares are at a loss. I want to know the amount of zakat due on them?