Framebuilders - cheap bike

i am looking to bike 245 miles this summer but i have never really biked before. never had a bike. never even thought about it until about a month ago and now i need a bike if i want to make the journey. im mostly looking at used considering i dont have enough for a brand new bike. so my questions are:
1) what is the best way to get a lightwieght bike??
2) should i build the bike or buy a complete one??
3) would craigslist be a good idea to get a halfway decent bike??
4) how do i know when a bike fits me good??
5) if im 5'8" and 126 pounds how big should my bike be??
6) is it possible to get a decent bike for under $400??

thank you for taking the time to read

1. weight is the least important of all factors really, anything from 18-25 lbs will be fine. Too light and you sacrifice durability.

2. buy the complete one, saves you the most time and money. Only build if you need specific list of components for whatever reason.

3. Yeah, craigslist is fine, you should know how to inspect a bike. Bring a bike-shop friend along if you don't know what to look for.

4. easy fit method is to stand over the bike and straddle the top-tube. Feet flat on the ground and lift the bike up evenly. Should have 0.5-1.0" clearance before the top-tube touches your crotch.

5. Depends upon your inseam measurement. That height would typically ride a 55-56cm frame.

6. Yeah, you can get darn good used bikes for $100-400 from Craigslist, thrift stores or garage sales. Ex racing-bikes with nice high-end componentry and decent frames too and they're typically only 3-5 years old in that price range.

alright. thank you for your help

Yeah I got me a carbon fiber road bike for $40 at Salvation Army

Seriously? Wow. I was actually looking at getting a second bike for the trainer, etc., and was strolling through costco.com. I may have to re-think my options.

http://www.carlselesky.com/