If your pressure is dropping to zero, you may need more than the T&P and expansion tank...the T&P may not be resealing, or you have another leak since it normally is set to relieve pressure at around 40# or so. Your boiler should say what its normal operating pressure is...typical is around one atmosphere 14-15#, and you'd set the pressure of the expansion tank before installation to the normal operating pressure.
Shut the boiler off. Relieve pressure by opening a drain valve. If it has an autofill valve, shut that off. You don't need to drain the whole thing. If the water is hot, you may want to wait awhile, since you'll probably spill some, and you don't want to get burned. If you have any valves around the expansion tank, you could close them to limit how much water drains when you take it off.
The expansion tank is probably full of water, so when you unscrew it, support it well, it will be heavy. The new one is mostly air, and will be light. Make sure to use either pipe dope and/or teflon tape on the threads when installing the new one. You should have enough strength with that tank to hand tighten, no need for any tool to install or remove.
The T&P valve likely has a drain line attached. To unscrew it, you'll need to first remove that. If you don't have any soldering skill, it is time for a pro. Same thing on it, the threads need sealant - pipe dope and/or teflon tape. You may need a pair of pliers or a wrench to ensure it is tight enough not to leak since it is nowhere near as big around. you'll likely need them to remove it as well.