Commerce

The best way to learn any programming language is to practice and practice. Both C+ and Java are among the most popular languages and includes object-oriented programming.
The first question that arises is that which one of these to learn first. Both the languages are similar in many aspects like both include multiple inheritances, polymorphism, abstraction etc. But there is a difference in the way these features are implemented in both the languages. Keeping in mind the syntax of both the languages and the way the features of these are implemented, it is better to learn C+ before Java. Moreover, C+ is similar to C which is the most basic programming language. After learning C+, one can easily switch to Java.
Now, what are the habits which make you good at programming? Be regular, understand the basic concepts, practice as much as you can, that's it. The first stage is to completely understand the fundamentals of the language and practising at the same time. The second stage is solving higher level questions to improve your algo. Get enrolled in HackerRank and similar platforms to improve your coding skills further to compete with many more programmers like you. And the last stage is coming out with something different with your programming skills.
Where to learn these programming languages? I would like to name you a few good websites for learning programming.
For C+
Learn C+ - Covers a large number of topics and a good website for someone who previously knows some programming language.
Codesdope - A very good website for those who wish to learn a language completely from complete beginning
Cplusplus - Another good website for learning C+.
For Java
Codecademy - You can learn complete Java but it covers only beginner level Java without going much into
Tutorials Point - Covers a large number of Java topics and is good to start with
Codesdope - Good to understand the concepts clearly for anyone
Hope it helped.

Yes, you can.
One thing important here is: Why would you want to do that?
After studying commerce, growth of commerce is huge.
But if your interest lies in science then you might shift. Why most of the people go for an MBA post engineering, B.Sc, science courses, etc. The answer is accounting, marketing, financial education is the key to understand any business.
It is good that you have studied the basics in commerce. If this has made any interest in you do stick to commerce for higher studies.
Else, yes you can opt for B.Sc but the cut-off for any college will be higher for you than the science folks in this case.
Try to make your decision considering various factors. Your interest, aspirations, what you expect from your life and job. How will this make you achieve your goal in the long run, etc.
All the best!