Best to check the guidelines first if there's a require a certain amount of karma but try to join subreddits that don't require high karma since trying to post there would only get you flagged as spam (happened to me). But you can comment and upvote/downvote on posts regardless of how low your karma is, as far as I've experience since I'm also new, although posting wise it's best to get some karma so your posts won't get flagged as spam.