If you already have a programming background, the rubydocs and tutorialspoint is the best way to get a head start. You can easily understand the purpose of each methods and its syntax
Once you have a decent knowledge about all the methods in Ruby, start practicing programs in Hackerrank. There are hundreds of problems in Hackerrank and within two months you can get comfortable with Ruby.
However, if you do not have a programming background, it is better to start from the scratch. It might be slow, but it will be effective. Some websites that teach you Ruby from the scratch are