I started "seriously" with 3D about two year ago and the most important piece of advice I can give you is to spend some time to carefully choose your 3D package and then stick with it: 3D softwares are quite complex to use and very complex to master and if you try to use too many different softwares, you will just spend most of your time learning them instead of learning 3D.
Cinema 4D is an excellent and well known package I would recommend.
Oh, and don't spend too much time with Poser characters if you want to produce microstock images: we are still a few years away from photorealism
I think that trying to produce simple microstock 3D renders (which are not rejected) is a good way to learn 3D basics as you can get very simple 3D scenes which sell very well. One of my best seller this year so far is a 3D render of an isolated milk box
Cinema 4D is an excellent and well known package I would recommend.
Oh, and don't spend too much time with Poser characters if you want to produce microstock images: we are still a few years away from photorealism

I think that trying to produce simple microstock 3D renders (which are not rejected) is a good way to learn 3D basics as you can get very simple 3D scenes which sell very well. One of my best seller this year so far is a 3D render of an isolated milk box





