i want to but a vps service but i have a few questions to understand wich really fit my needs.
How many websites may i set in a vps? what are requirements to run about 30 small sites?
May i set my server, for example, to use apache + red5 ?
Can you suggest some good resource to understand better the VPS and how to manage them?
May i resell shared hosting in my vps?
The requirements for a VPS are almost the exact same as a physical server.
Depending on your plan, some hosting providers will allow you to install whatever software you want on the VPS, others limit you to their particular selection(s). Reselling will depend on the license; it's common to see it go both ways.
We have absolutely no way of knowing the requirements for "30 small sites". If those sites were Google, Yahoo, Facebook, Microsoft, etc; the requirements would be insane. If they were 30 one-page static 'sites' then the requirements would be next to nothing.
How many websites may i set in a vps? what are requirements to run about 30 small sites?
All depends on number of visitors, if you have less than 100 hits a day, any server will be able to serve them
May i set my server, for example, to use apache + red5 ?
depends on the provider, some will allow you to install any software, OS and up, some will support only particular setup
Can you suggest some good resource to understand better the VPS and how to manage them?
a VPS is hardly different from a dedicated server, but they usually have some kind of Webmin preinstalled or available for small fee
May i resell shared hosting in my vps?
Check T&C of the provider, some permit, some do not.
How many websites may i set in a vps? what are requirements to run about 30 small sites?
That depends entirely upon the sites as Chris S. noted - if you are trying to do things as cheaply as possible and you are comfortable with systems administration, I would recommend the 64MB VPS Static Webserver guide at LowEndBox.com ... if you know you'll need Apache and MySQL, you will probably be looking at needing at least 256 MB of RAM as a bare minimum (again, assuming you're comfortable with systems administration).
May i set my server, for example, to use apache + red5 ?
A fully-managed hosting service should include systems administration assistance to install whatever you need - this is the recommended option if you are not comfortable administering the VPS yourself.
An unmanaged hosting service will drop you at the bash prompt. If you're not comfortable using bash, you probably shouldn't use an unmanaged hosting service (it will end up costing you a lot more time and money in the end).
Can you suggest some good resource to understand better the VPS and how to manage them?
If you need to learn Linux systems administration (and you have plenty of time on your hands) the first command you need to learn to use is man
As far as VPS hosts go, you might try shopping around at comparevps.com to get a feel for what different providers offer and how much you can expect to spend.
One thing you may want to consider (assuming you are looking at Linux hosting) is that OpenVZ virtualization makes overselling easy - it's comparatively difficult to oversell with Xen virtualization, so go for Xen (particularly if you plan to run Java).
May i resell shared hosting in my vps?
Read potential providers' Terms of Service. Note that many providers offer cPanel licensing for a nominal fee (if you plan to offer website/e-mail hosting, cPanel will make your life a lot easier).
