Google doesn't help much with that. It usually does, but I'm pretty sure you didn't get an HDMI capture card for a Hi8 tape! (or at least I hope you didn't, because it's probably not going to work) What is it really?
At any rate, you should be able to plug in the capture card, then add a Video Capture source in OBS, and tell it to use that card. If you're giving it a signal while you make that selection, you'll see when you've got it right.
Then if it doesn't come with audio already, add an Audio Input source, and point it to the correct thing also. Or make it a global audio device in Settings -> Audio.
Once you have both picture and sound, check the recording settings in Settings -> Output, including both the encoder and the recording folder. When you like that, click the Start Recording button.
Another note is the canvas size. For Hi8, use 640x480 for both the canvas and the output. Settings -> Video. Then make the Video Capture source full-screen. Right-click -> Transform -> Fit to screen.
