so for instance i have 100 sf of wall to paint. when i want to calculate durations using the sf data and the productivity / worker / hour data, how can i translate this 100 sf of wall to work?

for example if i know that one worker can complete 100 sf of wall in 8 hours, then does the work become 8 hours?

in other words,

1-does the work mean the amount of time required for one (1) unit to finish that entire task?

2-in general, when i have sf data, and productivity data, and when i want to translate this into ms project to come up with my duration, is the work column where I start? if not which column i start with? if work column is the place to start, then i enter resources available, assign resources and then it calculates the duration? is that the sequence?

No, the amount of work is the amount of effort for that particular resource. Split your screen to show the Task form in the lower pane.

Assign the resource using the Task form and enter the work in the work column. Click OK and Project will recalculate the duration required for the assigned resource to accomplish that work given the assignment units.

In the case of you specifying work, project will calculate duration by:

Work/Assignment Units = Duration