For beginner, you can do a simple market research to determine the demand and the competitiveness of your niche.
Demand analysis
Use google keyword tool and check the number of monthly searches using the exact keyword for your niche. Use long tail keyword like "dog food for toy poodle" instead "dog food" or "dog food for poodle". Be as specific as possible.
Competitive Analysis
Google search the exact keyword you use in demand analysis using "allintitle: your keyword". this way you can check how many people are already in that niche.
Divide the number of monthly searches you got from demand analysis by the number of results from competitive analysis. A ratio of 2 or more