Since you lack knowledge and skill, you wouldn't be a big help in OS projects yet, don't take that personal. So I woulld recommend you start with a project you are interested in. Something that would help your life and your friends, that can be an Mobile App with Web Tech (See later Link Collection), a Web System or anything else. Just try to build things which would help you and other people.
When you are done, take a look how you have done it, which problems did you face and did you solve them. In both cases, check out how other people have solved them or how to even avoid these issues completely. Take a look at the PSR Style Recommendation for PHP, LESS for CSS and especially read basic tutorials on MVC and jQuery.
You just lack knowledge in terms on how to solve things, so you ideally should start off with reading style guides on HTML, CSS and jQuery. If you want to, you can include things like LESS into the list of "What to learn?". Don't read only one guide, read multiple ones all over the net and try to combine them to something where you can say "Hey, that's useful" and apply it to your project.
Some things which you might find useful in your future:
http://blog.codinghorror.com/understanding-model-view-controller/http://cssguidelin.es/http://lesscss.org/http://getbootstrap.com/http://www.php-fig.org/psr/psr-1/https://angularjs.org/I used most of these resources and I guess you might find some of these useful too.