The most important thing is to focus on the potential customers. Below is how people conduct their keyword plan, so they will know which keywords to focus on as a part of SEO strategies.
Step 1: Brainstorming
Answer this question: "What services do you offer in customer's perspective?"
Step 2: Evaluate search volume
Focus on long-tail keywords.
E.g. iPhone cases are less effective than blue iPhone cases, and blue iPhone 4s cases are more effective than the others.
Step 3: Categorize by keyword
For example: model, brand, color, custom...
Tool to help find keyword