Hello! Ozeki C# VoIP SDK - A SIP SDK for software developers
If this is the site you've downloaded from, look for the link for documentation or download the manual.
The samples are provided to help you make a start. Pick the simplest sample and get stuck in.
Find some references for c++ etc. If you can't understand some of the code in the samples, then you can refer to the c++ reference/manuals to help you progress.
I'd also recommend looking for some video tutorials.
The first thing you need is an understanding of the computer language you are going to use.