For base LFS cars I recommend using already made templates available here:
http://www.barkingpig.org/msz/pages/kits.php
To answer your questions:
- you don't need to make wireframe, use what somebody else made or generate it from lfs itself
- wireframe is a image which shows shape of car on flat picture, you can see things like car shape changes, door hinges, handles, etc
- get wireframe and set it as one of top layers in your graphic program, you can then use it as reference by drawing shapes over it or just positioning things
- go to your garage (shift p) and click tab named "colors", at the bottom right there is a button "generate skin template", one of them is wireframe. Another useful one is black and white skin texture. Shaded skin texture is almost the same, it just has more details you could get with wireframe used with imagination. Experiment with every one and see for yourself.
The process of uploading skin to lfs servers so other people can see it is explained here:
https://en.lfsmanual.net/wiki/Skin_Tutorial