I made a Reddit account but I cant post anywhere because it says I need to have 200-300 karma and be active. The only way to get karma is by other people upvoting your posts so I fail to understand how I will ever be able to gather the karma if I cant post anywhere.
That's a restriction on the subreddit. You may have to make posts to other subreddits that you might not really care about before you will be allowed to post in that one.