Yep, I'm using Unity, modeling in Blender, and texturing in GIMP.
The first and most obvious tip when starting with 3D applies to pretty much any form of art: start with primitives and just shape and connect them until you have the complex shape you want. Need a pillar? Create a cube, add two edge loops, and scale those in. Bam, done! Need a robot? Jam some cylinders and spheres together and squish them around until it looks like a robot. In fact, if you push the concept of "primitives" into abstract territory, every single thing you do can be broken down into smaller parts, and those smaller parts are generally easier to manage. So, do it as much as possible.
Building on that, it's far more productive to just throw a bunch of primitive shapes around as quickly and sloppily as possible and then refine that later once you've settled on a final vision than it is to try building something piece by piece until it's finished. You waste far less time making stuff you'll just throw out when you change your mind, and your final product is much better as a result of the additional iterations it when through and the holistic vision it had before the refinement process.
Again, this applies to pretty much anything you could possible do creatively, be it modeling, writing, drawing, level design, or even planning. So it may not be new advice. But, it's always advice worth refreshing oneself on when starting new creative endeavors. Just get something tangible out there and worry about the details when the details actually matter.
The next best tip I have is to adopt an initial style that facilitates learning. Start with palette textures (images with nothing but solid squares of color). Just scale polygons down in the UV editor and stuff them into the squares to color them. This lets you focus on modeling first and nets you a nice and consistent cartoony style as a bonus.
you're comfortable with modeling, if you find that palette textures are
too limiting, try using gradients instead. Just grab vertices and drag
them around in the UV editor to change the color for that vertex. They
combine the ease of definition and updating of palettes with the
smoothness of vertex colors. To quickly project the mesh into UV space, select everything, press numpad 1 (front view) and 5 (orthographic), followed by space > type "project from view." The other numpad buttons can help you get other views.
If you use Blender, I strongly recommend learning the most common hotkeys and forcing yourself to use them as much as possible. Don't even use the transform widgets, since the hotkeys are faster, don't require holding a mouse button down (a huge plus for people like me with a faulty mouse), and also work in the UV editor. At first, it's a bit of a hassle to deal with hotkeys in Blender. But once you get used to them, the interface melts away.
Selecting all/nothing (A), Creating edge loops (ctrl/cmd+R in Blender) and extruding (E in Blender) are easily the most common actions outside of rotating (R), scaling (S), and what I refer to as grabbing (G) for easy memorization. Also, pressing X, Y, or Z limits actions to specific axes; shift plus X, Y, or Z is the inverse of that; and typing a minus at the end of a specific value is just as valid as typing one before. You can also press space and simply type whatever you're looking for into the search bar.
There are also a couple of preferences that make working with Blender easier. First, I suggest going into your preferences and swapping the right and left mouse button's actions. That inverted default is entirely ridiculous. Second, Blender defaults the z key to toggle between solid and wireframe mode. But I find that I'm much more likely to want to swap between material and wireframe mode. That can be changed by going into file > user preferences > input tab > 3D view > 3D view (global), scrolling down to the Context Toggle Values that are assigned to Z, expanding it, and changing the first value from solid to MATERIAL.
Finally, I recommend creating a clean startup file by deleting everything from the hierarchy and going to file > save startup file.
That's about all I can think of at the moment. If you have any specific questions, let me know!