Hi and welcome to the forum!

The short answer is that, if your girlfriend accepts your proposal, yes you will have to return back home to apply for a fiancé visa. The gory details are here:
How Do I Bring My Fiancé(e) to the United States? (She will have to start the process in motion by "petitioning" for your visa application.)

But - the good news is that there is no reason you can't start looking for a job in the US before you go over to propose. Explain your situation in your cover letter and see if you can at least schedule an interview or two while you're on your "proposal trip." Permission to work comes pretty quickly after you have returned to the US on the fiancé visa and if you already have a potential employer waiting to hire you, so much the better.

The US immigration people steadfastly refuse to even estimate how long the visa process will take - in part because there's quite a bit of collection of documents on your side (and for your fiancée) that can take weeks or months, depending on your situation. You also have to have a visa interview at the US Consulate and these can take a while to schedule. (The interview itself lasts about 10 minutes.) Count on at least 3 to 6 months - although if all goes smoothly, you could be pleasantly surprised.
Cheers,
Bev