For full time and part time loads, it is based off of how many credit points you are doing that semester so if you want to be considered a full time student you will have to do a certain amount of credit points. For example at UTS to be a full time student you have to do 18 units per semester to be considered a full time student otherwise you'll be considered a part time. Also credit points are basically how much each subject is worth as you need a certain number of credit points to graduate with a degree.
For choosing subjects/electives I can't speak for all unis however, typically you would follow a recommended course structure as some subjects (at least for UTS) have pre-requisites meaning that you must complete the pre-requisite subjects before you're allowed to commence in a subject. While it is possible to deviate away from the recommended course structure it's not typically encouraged as you'll potentially encounter issues with pre-requisite. For choosing electives I'm not sure for all unis but at least for UTS you're given a list of electives and you choose a certain amount from that list depending on how many your course is allows you to choose.