Kiluad

I'm trying to learn how to code the proper way. There's a few things im worried about,but overall im worried about clogging up the server. so what should i keep in mind while coding, to avoid this? I'm thinking, keep insert and update to a minimum? as in, when possible, use session to keep track of data until i absolutly have to add that data to a database? is that a correct thought? what about once the data is in the database, how much trouble is it to ask for the selected data inside a database? should i be worried about overdoing request of data from databases? on the other hand, should i be worried about having too many sessions travelling around with a user. why? Im just asking to get a better feel for how to do things the proper way, i would hate to have a website built, only to find out that what i built is only a bat, that would bash the hell out of a server i wanted to put it on.