Hello to everyone!

I am completly newbie with this script language, I knaw the basics but I want help. If any one can help me with that or If he/she can write all of it i will appreciated that. Here is what I want:

When a contact send me a message that begin with the letters RDS and then the message msg plus automaticaly add a line in a specific TXT file that contains

Contact_name: message message message .........

then start a countdown 10 minutes, and after 10 minutes pass go back and delete this line.

This will be continue for all the contacts in the same time in the same file.

in other words each message that start with "RDS" will be stored in this file for 10 minutes and then the line will be deleted.

The txt file will have 2 or 3 lines that must be there and never deleted. 

If someone Knows something please let me know.
If you have more questions please ask.

Thank you and sorry for my english.