save and load a record

I know the question sounds simple but actually it is not for somebody who programed with Ruby on Rails before just doing @user.save to save and User.find(params[:id]) to load it. For example I read somewhere that I havet to give NULL to the $edit-Values I saved to database, I am very confused about so many different steps to save a record. I could'nt find anything in the Handbooks about it:

1) How can I load a record so the values are the default values in the upcoming edit form, if the form is part of the user account edit form in a subtab (the subtab and form is already programed).

2) Do I have to consider something so the output data is save to be displayed without malicious code?

3) Standard way to save the record when somebody clicks on the "Save"-Button - I do it with a hook_user and case 'update' or 'submit'?

4) How to do in a save way so no malicious code will be saved?

As there seem to be various lines of code and in the modules I couldn't figure out a standard version as the code is written on a different place in each module in a different way, maybe somebody experienced with drupal could write the few lines of standard code here making it much easier for people to get into module development.

Thanx a lot,
Marenz