Thaumaturgy? Kill some ghosts or whisps, and use their drops for crafting.

Keep sacrificing the runes at the alter. When it says it is not accepted, that doesn't seem to really mean anything. You can skill get an upgraded rune back, and you can still gain theurgy skill.

To train theurgy get yourself some refining tools and refine every rune you receive down to 0.2. Invoking also gets skill-ups but at low theurgy you won't be receiving runes very often.

Sacrificing at an altar is to upgrade them. Pay attention to the rune icons at the bottom of the altar; the bars at each tier fill up depending on how many you are offering. I think ten of a lower tier to get one of the next highest tier plus some extras seems to be the going rate.