I'm new to performance testing, please recommend a tool which is good
for a beginner
The amount of time it takes you to become effective will be entirely dependent upon the maturity of your foundation skills which you are bringing to the discipline. Very good, robust skills and you can be effective in 90 days, with a mentor. Very weak skills and no mentor and you will languish for years. There are people in the market who have five years of experience on their resume with few useful skills
A tool is not the discipline, any more than knowledge of a particular accounting software package makes you an accountant. If you are just looking to dabble, to understand better the challenges of the role, both tool and non-tool related (tool is 5-15% of the job) then Jmeter is a fine place to start.
If you are going to use a commercial tool then honor their license requirements related to training and non-production use of training software or you will need to ask yourself, "If I am willing to compromise my ethics to violate a license agreement then why would anyone trust the output of my tests and my recommendations related to changes in the system to increase performance?"