There are many free 3D models available on the net and I'm sure there are aircrafts among them. You can use the free program meshLab if you need to convert between formats.

There are several libraries that can import 3D models into Processing. See the import/export section of the contributed libraries or for example ToxicLibs, which has an SLT reader.

You were advised to take a look at meshLab, although I am not sure if it allows for modeling. AutoCAD is a costly payware, so I suppose it is out of the picture. There are a number of free 3D modelers, like Blender or SketchUp. I just found in my bookmark a good list: http://www.hongkiat.com/blog/25-free-3d-modelling-applications-you-should-not-miss/ The hardest part is to choose one...
There are also sites offering free 3D models, of various quality, but worth visiting, I think. I have also a list: http://www.hongkiat.com/blog/60-excellent-free-3d-model-websites/

See http://plane.turbosquid.com/3d-Models/3ds/max/xsi/c4d/obj found with a simple Google search: free 3d aircraft model
There are more...
Again, there are free tools to convert 3D models between various formats, targeting one that can be understood by the libraries mentioned above. After all, you have to display the model in Processing, at the end, no?

" ive never done this before"
Neither me... Have you tried meshLab, as advised? (I haven't...)
The problem with 3D is the multitude of formats, perhaps even more than 2D image format... Every software seems to come with its own.

All right, adding to the information above. The following steps decribe how to download and use a model in Processing. For clarity, there is no modelling involved. If you want to model yourself, check out phi.lho's link to free modelling software.

STEP 1
Download a free 3D aircraft model
Some example links...

http://artist-3d.com/free_3d_models/dnm/model_disp.php?uid=271
http://artist-3d.com/free_3d_models/dnm/model_disp.php?uid=1013
http://www.free3dmodelz.com/3d-aircraft-models/3d-airplane/
http://www.free3dmodelz.com/3d-aircraft-models/3d-fighter-jet/
http://www.free3dmodelz.com/3d-aircraft-models/3d-classic-airplane/
http://thefree3dmodels.com/stuff/aircraft/f_5e_tiger_ii_aircraft/15-1-0-411
http://archive3d.net/?a=download&id=6264
http://archive3d.net/?a=download&id=7196
http://archive3d.net/?a=download&id=6712
http://archive3d.net/?a=download&id=6508
http://www.quality3dmodels.net/viewmodel.php?id=180
and last but not least... 7 pages of free aircraft models @ Turbosquid

STEP 2
Importing into Processing
Most of the models above are in 3DS or OBJ format. There are different options...

OBJ > Import via OBJLoader or OBJ_Import library.
3DS > Import via MRI3DS library.
OBJ/3DS > Open in MeshLab, save as STL, import via ToxicLibs.

So you don't need to download and do everything. Just download a suitable model in a suitable format ( step 1). Then choose a route to get it into Processing ( step 2). Personally I have used the route of converting 3DS-to-STL in MeshLab and then importing via ToxicLibs, which worked all right.

Well, here is a very basic example of getting a 3D model from an OBJ file into Processing. All of the 3D model files should be in the data subdirectory of your sketch (obj, mtl and textures). Are you sure that your downloaded 3D model is in the obj file format and not 3ds or something else?

1. // Simple OBJ_Import example (also requires peasycam library)
   
2. 
   
3. import processing.opengl.*;
4. import objimp.*;
5. import peasy.*;
6. 
   
7. PeasyCam cam;
8. ObjImpScene scene;
9. float objectScale = 15;
10. 
    
11. void setup() {
12.   size( 800,600,OPENGL);
13.   scene = new ObjImpScene(this);
14.   scene.load(dataPath("spheres.obj"),objectScale); // where spheres.obj is your object's name
    
15.   cam = new PeasyCam(this, 100);
16.   cam.setMinimumDistance(1);
17.   cam.setMaximumDistance(100000);
18. }
19. 
    
20. void draw() {
21.   background(200);
22.   perspective(PI/3.0,(float)width/height,1,100000);
23.   scene.draw(); 
24. }
25. 
    
26. void stop() {
27.   super.stop();
28. }

p.s. the examples from the OBJ_Import library should work, apart from a bufferedImage-issue in one of them, so something is going on there also. First make sure the library is installed correctly.