You can apply thru your employer or you can do it on your own. You need salary letter from your employer (and there is another letter issued kind of no object..not sure..you can check with yr employer). All you need is your Father's PP copy and your Qatar ID. Take all these documents and go to immigration office. They will give you a receipt. After 2- 3 days, you can check the status of your application and print the Visa online thru MOI site.

All Visit Visa will be issued for one month initially, and you need to take your father to the medical commission for the medical test. after 3 - 4 days post Medical test, go to Immigration office, get the extension max upto 6 months depends on the age. As far as I know there is no fixed age parameter on this.

Hope I have give a decent idea on the process.