How much disk and memory is my Elasticsearch using and occupying?

I'm trying to determine my hardware needs. So I want to find exactly how much memory and disk space am I using and occupying.
What are the commands to use?

You can see this in Monitoring in Kibana pretty easily.

Otherwise the _cat/nodes?v end point will help.

No I want to take a decision how much extra disk to rent. Now I'm using around 50GB for each of my 2 nodes and I need to expand the disk. How can I know how much more disk do I need?

If you run

GET /_cat/nodes/?v&h=name,du,dt,dup

You will get du = disk used, dt = disk total, dup = disk used percent
Example

name                     du    dt   dup
instance-0000000075 146.8gb 240gb 61.17
instance-0000000074 146.1gb 240gb 60.88
instance-0000000073 167.8gb 240gb 69.96

This should help guide your decision.