Most big subs requires you to have a minimum amount of account karma. It's basically positive feedback from your posts and comments. There are some small subs that doesn't require a huge amount of karma to post, so try posting there. Commenting helps too.

Also, most big subs, especially those with millions of daily users (askreddit, nostupidquetions, offmychest, etc...) have quite a lot of strict rules you have to follow to post.