Renewing visas

[eze 0]

im a little confused about this (well, more than a little), so hopefully you will tell me..in detail.

when you leave the country after 90 days, how long do you have to leave?  does it matter at all where you go?  do you have to apply for a new visa, or an extension, or what?  and do you have to do this before or after you leave the country?

also, how long in advance to you have to apply before you go to thailand (19yrs old, canadian)? ???

one more thing..which of the following countries require you to have a visa if only staying a couple weeks:

cambodia, myanmar, laos, vietnam, malaysia, indonesia, india, tibet, nepal, bangladesh.

thank you