Hi there. You won't know me, because I don't post much around here, but here is some
street cred.
First off, Sketchup is designed for producing models for visualization - how it looks at the end is all that's important. It is
not a tool for creating working 3d game models, it just doesn't have the capacity or adaptability for it. If you want to work on this, you will need a serious 3d modeling program like Blender, 3ds Max or Maya.
Just to back this up, here's an example of a model made by someone else in Sketchup, which they brought to me as an .obj file which I imported into Blender:

Note especially the absolute mess near the right side of the image, and the cylinder near the middle. This model looks nice like
this, but is absolutely horrendous from a usability point of view. I was able to
recreate the model (with most of the detail intact) with less than half the polys of the original.
Anyway, the point is that Sketchup is not an appropriate tool for this kind of work. It can't handle the .md3 format that Tremulous uses, nor does it (to my knowledge) do model animations or UV mapping. You see, when it comes to producing game models, creating the mesh isn't even half the battle (it's sort of the easy part).
A quick word on the 'professional' 3d modeling programs - if you do a quick search for screenshots of 3ds Max and Maya, you'll notice that both have tool buttons around the edges of the modeling window; Blender does not do this. This makes these two programs more visually intuitive and therefore easier to learn quickly. In contrast, all of Blender's important modeling tools are bound to various keys, so the only way to learn them is through references and practice. However, it's very important to realize that in 3d modeling,
all of the important work is done with the cursor! This means that in 3ds Max or Maya, if you need a tool to edit the mesh, you must stop what you're doing with the cursor, move the cursor out of the 3d window to select a tool, and then back again to use the tool. In Blender there are no such limitations, and you can simply choose and apply any function at will. The practical upshot of this is that you don't have to stop using the cursor in the 3d window, and so your work on the model proceeds much much faster. It's possible to build very complex meshes in Blender in a matter of minutes.
tl;dr - moving the mouse out of the workspace to get a tool is slow and bad, using the keyboard to access tools quickly is fast and good!
If you want to learn 3d modeling, you most certainly need a decent 3 button mouse (optical or laser, not a ball!). You need both the extra buttons (for moving around in 3d space and manipulating your model) and the pointer accuracy that a mouse provides. A touchpad
will not cut it. If your laptop doesn't have a full-size keyboard, it's probably also worth investing in an external numpad as well, because in Blender this is used to precisely control your viewpoint.
I know that's a lot to read, but if you're still with me then I'd like to offer to help you more. I spent a long while learning to use Blender and then learning to use it in conjunction with Tremulous, and unfortunately a lot of the information is spread around or just generally hard to come by. If you're interested, I can be found on
irc.freenode.org in either the
#unvanquished or
#knightsofreason channel (unvanquished has a couple other experienced modelers in it as well). I can get you started learning the basics of Blender, generating clean and efficient meshes, importing and working with Tremulous models, and then model animation, proper UV mapping, and drawing your own textures using the GIMP. I can also be found more readily on our forums at knightsofreason.net, but I find that a real-time discussion through IRC is much more useful.