You need to learn how to model with a 3D modeling program, i would start with a Blender 3D since it's free. There are are heaps of helpful tutorials on youtube or on the Blender site which you can check out. I would download the 2.6 versions of Blender.
If you go here:
http://www.ogre3d.org/tikiwiki/OGRE+Exporters You can find the exporter you need there.
Hammy has a tutorial and has links to programs you will need:
feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=11025.0
You will definitely need python 3 and the ogrecommandline tools to export to .mesh.
I am not too sure how to make particles from scratch, i would ask Glave about that