How do i make a flippin model?

well i have been on gmod for a while and i wanted to make my own model. but the problem is i have no idea how to make em. i have been on blender for a while and i know how to use it and what not but im not sure on how to turn the mesh into whatever file u need to so… any help here? and before u yell at me for posting it in the wrong place or anything im kinda a noob i never use forums SEARCH