I want to post comments and questions on this site, but I do not have a 50 reputation. How can get a 50 reputation so that I can actually use this site?
As Al has mentioned, anyone can post questions to the site, regardless of reputation. Just about anyone can answer as well. The exception to that is "protected questions," which one must have at least 10 reputation to answer (not including the association bonus, but that's not applicable in your case).
In addition to either of those, anyone can suggest an edit to a question. If your edit is a useful one and is approved by the community, you can earn 2 reputation points. Poor edits are almost always rejected, so try to improve everything you can in the post before submitting it.
See the Meta Stack Exchange post Why do I need 50 reputation to comment? What can I do instead? for more details as to why the sites require 50 reputation points to comment.
However, history and experience have shown that the downsides of allowing everyone to comment are far greater than a few good-quality comments:
There are big problems with spam. Automated filters cannot catch all of it.
Even among the real comments, most would either say "I have the same problem" or "I agree". Such comments do not add any value, and have to be manually removed.
Comments are very painful to moderate. Stack Exchange sites have a process of community moderation (voting, flagging, review queues) that works great for questions and answers, but not so much for comments. Comments cannot be downvoted or closevoted, nor searched (nor do we want that).
Comments are second class citizens on the Stack Exchange network, not designed to hold information for all eternity. They may get cleaned up at any time. Generally, truly important information should be incorporated into an answer of its own anyway.