Selecting the relevant papers for a survey paper I have to write?
The answer may differ from field to field.

First of all ask yourself "What is the central question" or the goal of you paper?

Do you want to give an overview on all aspects of the theory / concept? Or is it more about the central aspect of the theory / concept and extensions / applications are not that interesting? Or something else?

Then think about the story you want to tell. If its about the theory itself you might want to talk about the development of the theory, central aspects, proofs, and maybe some applications or extensions at the end. 

If you want to give a general overview, try to create groups, are there some main movements, aspects, fields of application? Where are the links between these things?

Then select your paper according to this "central story".

While writing think about the audience of you paper, which concepts are common knowledge? Which are important to explain? Which depth of information is appropriate? 

And, of course, try to get more feedback: Read the feedback of your old paper, then read your paper and try to find the flaws. Ask the person who graded you to give you some advice. Read some paper of your fellow students which got good marks, find out what you like about them.

in short:

1. Identify the central question of your paper
2. Create a structure / central story    
3. Think about the audience
4. Fill the structure, create links between your sections.