I am student with freelance skills but how to switch to opensource?

Which open source projects are you using and you like? Try to contribute to them.

Start sending bug reports that are very useful and dometimes underestimated. Look how/where they fix them and the next time try to send a pull request (or another bug report).

Fixing typos in docs is also a good starting point.

If you want to send a new feature, start with something small, like a weekend project. You never know if the mantainer will like it, or if they are morons or if they have very weird code rules.

Check out the FOSSjobs website and wiki:

https://www.fossjobs.net/ https://github.com/fossjobs/fossjobs/wiki/resources

Especially look at the internship programs and choose an area that has a significant number of employers, such as the Linux kernel.