Wow! Now that is a broad and all encompassing question!
I would do a few google searches on the following topics:
Benefits of partial water changes in reef aquaria
water in the reef aquarium
How live rock works as a filter
I would start with these searches and go from there. These helped me along with looking up Carey's 125g build thread on this forum. This thread helped me immensely!!
As far as the tank, I would go with the 75g as it is only a few inches wider than the 55, but you will thank me layer!
Hang in there and I'm sure others will chime in as well...