T2 of 89X52

"how to write T2CON, T2MOD"

#include <REGX52.H>

void write_to_t2con_and_t2mod( void )
{
   T2CON = 0x01;

   T2MOD = 0x35;
}

You need to start by reading the uVision Getting Started Guide, and working through the example projects in it.

This will give you a proper introduction to the tools, how they work, and how to use them - rather than just jumping-in blindly at the deep end.
(The uVision Getting Started Guide is available on the 'Books' tab in the 'Project' Window; The 'Books' window is also available via the 'Help' menu; failing all that, search for GS51.PDF in your Keil folder)

You should also look at the example downloads on this very site, and included in your Keil\C51\Examples folder