Green wood is huge fun to turn, because it is very easy and a sharp tool will leave a very good satisfying finish.
Carbide tools are not necessarily scrapers. Mine can get really longs shavings out of green wood and give a good finish.
The main problem with green wood is that once turned will very rapidly loose moisture and crack. There are many, and very different, approaches to try and get around that. Some will turn a bunch of bowls roughly and then let them dry very slowly in paper bags or newspapers. Sealing the end grain with PVC glue helps slowing drying even further, so less chances of cracks, which anyway do happen. If you turn in a shed or other humid place, you could leave them there for a few months and then let them dry further indoor. The environment determines the moisture content in wood, so drying in a not to so dry shed, finishing turning it and then bringing the piece inside can lead to it cracking.
Others turn green bowls with very thin walls, which takes skills. The result is that the wood will still move when drying, so the shape will change, but it might not crack. Once I read from a turner who did the above, but the would also put the piece briefly in a microwave, while still wet, and bend it with pliers while warm, fashioning into creative shapes. Never tried that, and using the kitchen microwave would probably lead to divorce, but it sounds like fun.
If you do not care about your first experiments to crack later on, by all means use green wood for practising though. It is fun to work with.
Just, if you are given logs which were left outside for a while, make sure you are not turning anything with cracks. It can break apart while turning and that is dangerous. Same with spalted (rotten) wood. It can make for really nice figures, but it is fragile, can be hard on the hedge of your tools and some molds are quite bad to breath.
Marcros suggestion to try softwood posts is pretty good I think. A while ago I bought some to spend a few weeks turning only with a skew, turning mainly eggs, and I learned a lot. I am still bad at it, but much better than before at least. Soft wood like pine turn easily but also marks easily when your tool slips, and requires very sharp tools and decent tecnique to get a reasonably good finish. If you learn how to do that, you will probably become quite confident when working with harder woods.
And, as a completely unrelated and unrequested piece of advice, if you are starting is better to turn a lot trying to get right one aspect (one shape, one type of cut, one tool), than to to turn less while trying to get it all perfect.
Actually, I read somewhere interesting about this. It is about pottery, but I think it applies to turning too.
A college pottery teacher once did an experiment. He told half of his class that their final grade would be about quality. They could submit even only one piece, but if that was high quality as for shape, design and finish they would get a good grade.
He told to the other half of the students that their grade would depend on.. weight. They could submit as many pieces as they liked, and the total weight would make their grade, or break if it was not enough.
One would expect the first group to submit the nicest pieces, given they could focus on one single idea and make it perfect, and the second group to produce a bunch of so and so stuff.
But in reality the second group did better then the first. There was a bunch of crap of course, but most "grade by the pound" students submitted higher quality work than the other group.
Essentially, if you get good enough to replicate the same shapes and cuts precisely and confidently, it will not matter a lot if you come up with a good shape while practising on crap wood. You will be able to replicate it with a piece of good wood. Lots of practise can help taking luck out of the equation.
