question on making an offer... (newbie)

[Tyler Helton]

if I see a house I would like to buy, then I plan on tying it down. if I want to tie it down, then I need to make an offer. my question is: at what point do I apply and wait to get approved for the loan I will need? do I need to have the loan prior to making an offer? or apply for the loan after they accept the offer? 

also, after they accept the offer, is this when the due diligence period begins? 

and if so, can I negotiate the length of due diligence or is that predetermined? 

lastly, what are the stipulations called that are presented with the offer that would allow me to back out if necessary? and in those stipulations, can I allow for myself to back out at any point during due diligence, or is that already allowed, or....?