[cotyzor]

How I can calculate how much RAM will need my script to run ?

[RajatPawar]

Negligible and nothing that you need to worry about, unless you use Mapandreas which uses 75MB of RAM.

[cotyzor]

I want to know, because I'll buy an VPS and I must know what configuration I must purchase ...

And CPU usage how I can calculate ?

[iggy1]

You can launch your server and look at your task manager for mem/cpu usage. The amount will differ when players are connected though so keep that in mind.

There is also the Performance profiler plugin, which will give you much more detailed information on performance of your script.