Newbie: Objective Function
The objective function is the mathematical equation which combines the parameters in a useful sense. For linear regression this is a simple linear combination of weighted features. 

You will need to choose which objective function is best suited for your problem. Then based on the function you selected you will know what are the model parameters that will need to be fit using an appropriate training method. 

One of the skills of the data scientist is to identify the objective function which has the best likelihood of success given a problem statement.