Refer to the manufacturer's operation manual. These functions are usually described in a lot of detail therein, and the descriptions will cross-reference the P&ID for the specific machine in question. That's a lot more detail than we can possibly provide here. If you have more specific questions than "What does it do?" then please post them - we do expect some basic effort first though.

References:

"Steam Turbines for Modern Fossil-Fuel Power Plants" by Alexander S. Leyzerovich
"Power Generation, Operation, And Control" by Allen J Wood and Bruce F Woolenberg
"Operation and Maintenance of Large Turbo-Generators" by Geoff Klempner amd Isidor Kerszenbaum

Some of those might be at a level beyond your current level of understanding, but there is a wealth of knowledge captured in these texts.

Kavi14,
most utility plants have some training material. I am surprised informing you about its location was not part of your new hire introduction.
to simply answers the question I can.
the steam turbine's gland steam condensor is to prevent sealing steam from exiting the turbine casing and remove the air mixed with the sealing steam, returning the condensed sealing steam back into the cycle

I don't know drain cooler, could only guess

turbine startup is fairly complex and especially unit specific
in general, place support systems in service. this includes lube oil, hydraulics, sealing steam, vacuum etc.
control the heating of the turbines metal as limit the rate of change. this is where units vary greatly based not only on the turbine but the plants configration

3. How to startup the turbine?

Well, there's the recommended start-up procedure, and then there's what actually happens.

That's why engineering automate the controls: the computers make the decisions the same way each time, and it avoids operator 'creativity' if we lock out the option for manual override.

just realized, computer automated operation sounds like it would take the "fun" out of commissioning an unit .

Ha-ha, I didn't say Engineering weren't allowed to be creative.

This is how I tend to envision control room creativity...

The start-up sequence for a stand-alone ST is not the same as for a ST in a single-shaft combined cycle (SSCC).
And in a SSCC it is also different if there is, or not, a clutch to disengage the ST

And it also varies from model to model and from manufacturer to manufacturer, depending on the construction type of the rotor (bolted, welded), sections (HP, RP, LP), etc.
too many variables, but in general:
first: pre-heating of the unit
second: temperature matching/stress control
and finally loading to full load

saludos.
a.