How do I get a visa and passport?
You get your passport at your local passport or government office id say? We have local passport office in Dublin in our country and we apply for ours there. Im assuming it may be the same in your country
Before you get a visa it depends on what your purpose of travelling to Japan is. If it's work then you need sponsorship from a company before applying for visa. If it's for travel and tourism then you are issued a 90 day tourist visa upon arrival (some countries get 6 months).
how much does it cost to fly there and back?
Depends on the airlines in your country that service from your country to Japan. Check any airlines that are familiar with your country regarding price
will I need a guide or could I go alone?
I don't know of any guide services, their may be tour guides operating but you can always do it alone. If you have a decent travellers guide then you can make your own experiences by planning ahead
How much are cabs and hotels? over costs and essential things I will need
Both are very expensive. For accomodation i recommend youth hostels. Used them all the time i was there and had no problems with cost or quality of service. Usually cost in or around 2000-2500 yen a night. Hotels you could be paying anything from 10,000-30,000 yen a night.
Also how do Japanese people feel towards big people? I am 6'4 and 280 lbs. =]
As a foreigner you will stand out somewhat and get some stares, like yourself im a tall and broad enough guy and stood out that lil' bit more, hence you might get some stares moreso. But Japanese people overall are very friendly people.
Best of luck, any more questions fire away