We have like 5 of them? Petty, Lesser, Common, Greater and up to Grand. Is that right?

So what do I need to capture the last 2, are humans enough?

I had a whole bunch of variety gems, and when I used the spell, even common and greater seemed to be filled up with wolves and crap and didn't fill up the gem properly.

Likewise, I tried it on humans or similar, but the spell doesn't bite! So what am I doing wrong?

You need black sould gems to capture a humans soul. You can buy them from Enthir at the College of Winterhold or Falion in Morthal. Necromancers sometimes drop them as well. For Grand Souls you usually need the souls of high-level boss creatures like Mammoths or Falmer Shadowmaster. But if you need Grand Souls, it′s far easier to use Black Soul Gems, as Black Souls are MUCH easier to obtain.

http://uesp.net/wiki/Skyrim:Souls

To empty a soulgem, drop it on the ground, and pick it up again.

Wow! Thank you Would never had known that.

And thanks too Raliel, but I haven't really gone there yet, but I keep it in mind.

So Common gems would be Bears and Trolls? Maybe Tigers too

Lesser...erh dunno, everything is small after those up there.

Ah well, practice will tell.

Thanks for your ideas.

Here's a http://www.uesp.net/wiki/Skyrim:Souls of soul sizes.

I've found black soul gems are much easier to fill and at mid/high levels (over 30) I seem to be able to find plenty of them.

Other than mammoths and Draugr Death Overlords, the only consistently Grand souls I've found are from named spirits. Anytime you face a ghost or aspect or whatever with a proper name, try to grab it.

There is another type of useful soul container other than the five soul gem types and the black soul gems.