yes, if there's a working login function on the website then it's already a dynamic website.

as for the scripting, its best to understand how the whole website works before just diving in, editing or inserting your own stuff because for all that you know, the insertion or editing might cause an error and as you progress on improving on it you might be making the source of the website even worst.

so my advice would be to take some time and understand fully how the website works with the source that it's running then only insert your solutions or scripts.