The first program many people have used when beginning to 3D model would be SketchUp. Compared to the other free programs it has been around for a long time and still is a relevant program. It is a fairly easy to use piece of software and is well documented because of it’s popularity. Sketchup has released a series of tutorial videos to help you learn SketchUp if you are brand new or need to refresh on the basic. There are also a variety of forums and groups that are happy to help with specific questions you may have about SketchUp.
One thing to note about SketchUp is that although it can export STL files, some versions have a tendency to create non-manifold 3D models when they are exported. This can make them unprintable depending on the severity. With careful modelling this is avoidable but is definitely something to be aware of as it may be the cause of any weird behavior when printing. We find that using Netfabb in conjunction with this can help fix most non-manifold models.
SketchUp is a worthwhile tool to learn because it can be useful for almost any project you want to take on. If you are dedicated to making only mechanical parts or only artistic pieces there is generally other software that will better fit your needs. If you are new to 3D modelling, then overall Sketchup is a great piece of software to try out as it does introduce you to 3D modelling in a user friendly way.