"Cannot allocate memory" when upgrading

Hi,
I have some basic questions about creating a swap file -

1.) since i run a 2GB ram server, what should be the size of the swap that i create, and why?
2.) How will creating a swap file affect my server? If i create a 1 GB swap, will 1 GB be reserved just for the system processes for Discourse?
3.) As the forum grows, will i need to increase the size of the swap? What will determine how much should it be increased? And will the method to increase the swap be the same?

Many thanks!