The best option is to find someone who actually knows and understands what they are doing to show you.
The second best option is to get the exact model number, find the manual online (very easy - that make is well supported), download it and read it. You should find out how to fill the system and to what pressure and what the state the boiler should Benin (probably off and cold)
I THINK the valve with a slotted head is probably.some kind of shutoff valve. To be open the slot should PROBABLY be in line with the pipe and shut with the line across the pipe. Remember this is a GUESS, based on your limited description.
Get professional advice and, if you do DIY, do it when an emergency callout won't cost the earth
Fggio, September 2019
