No problem, we all were new at some point in time
The company will usually tell you and have it expressly visible in their description of the VPS.
This is because the server landscape is (for good reason) dominated by Linux, and BSD. Both OpenVZ and Sentora are Linux software packages (OpenVZ is for virtualization, Sentora appears to be a control panel for various Linux distributions).
You can either try to read up on server administration for Linux (for example starting with Debian or Ubuntu Server, very good tutorials and much help is available.) or easily find a offer that uses Windows Server licenses. Although they will usually be more expensive, simply because of the need for a separate Windows license.
Additionally, most (but not all) major server software was designed to be used under Linux/GNU or some BSD rather than Windows. So, depending on your use case, you may have more long-term satisfaction using Linux, although with a steeper learning curve if you are not already familiar with it.
Please also make very sure to be tight on your security - no matter what OS you end up using. Make sure to use a well-configured firewall, ideally some kind of rate limiting / HIDS (fail2ban, OSSEC, etc.), virus scanners if appropriate and protocol dependent filters (e.g. not acting as a open relay mail server, or a open dns resolver). Also read and follow the TOS of your future VPS provider. This is because you absolutely don't want to get your server compromised by malicious third parties.