Hi redostrike, welcome to the world of modeling. I too used sketchup before, and while some people can make some good stuff in it, you probably wont get very far into modeling with it.
This is just me personally, but i think a solid foundation into one of these three (free) modeling programs.
Anim8or
Pros:
Easy interface
easy for good workflow
Cons:
Not the worst rendering, but not good
bad uv mapper
alot of mesh errors for no reason (when other programs it wouldnt)
and bad subsurf tools ( highpoly making
)
But despite all those cons, i used anim8or for probably a year and a half, and i made some pretty decent stuff, but now i have a firm grip on the basics and thats what you should do to but sketchup is not the best way to do that.
Blender
Pros:
Good Uv mapper
Good subsurf
Edge loops ( very useful tool )
Sculpting tools (very good in 2.53)
So many more i dont know yet.
Cons:
Not friendly if you dont know what tools do
Mainly hotkeys (the reason for this is fast work flow)
my biggest agitation is the mouse1 doesnt do anything except move this life jacket crosshair thing wherever you click getting it in the way hahah. Everything is selected with mouse2
But some improvements are that theyve added customizable hotkeys (in the new beta, errr or one of the ones coming out soon)
Im using blender right now because it is 3ds Max *Capable. Some pretty amazing stuff it can do.
And finally
Wings 3d
im not going to do a pro/con list as i havent used this one extensively.
But what i can say is, it has a very easy layout to follow.
A nice big viewport, complimented by uv mapping, some say wings 3d has the best uv mapper but i never really could figure it out.
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I almost forgot, if you are in highschool (im pretty sure) you can apply for a student version of 3ds Max, which is AWESOME since that is one of the standard programs in buisness.
The only thing student version means is you cant sell models made by that.
But, that should definitely not be a big obstacle to overcome. Unless you endup absolutely needing to use max in which case you need 3,600 dollars.