Is your current disk set up one where OpenSUSE is the only OS on your computer ?
If it is, then all you really need to do is boot from your Mint USB, and install it. When you're asked whether you want it to be a sole install, or installed alongside an existing OS, choose the latter option. The Mint installer will automatically partition things for you.
You'll be asked how much disk space you want to give it. If 2 is as many OSs as you're ever going to want, then 50% of your drive space will split the drive equally between the 2 OSs, but to be honest you can allocate the 2nd OS (Mint) as much or as little as you want (provided there's enough for it to function properly).
Personally though, were I you, I'd get properly familiar with OpenSUSE first, before I started looking to install another OS alongside it.
Dual-booting is not without risk, and although the method I've explained usually goes without any major problems, that does not mean it always does. So if you're not technically savvy, then I recommend you avoid dual-booting until you've got a bit more experience.