Are the people joining your community new to the topic, new to the idea of participating in a community about the topic, or simply new to your specific community?
Let's begin with new to the topic. These are people that have only recently become interested in the topic. e.g. I join CoinTalk because I've recently become interested in coins.
If most of your newcomers are new to the topic then you need to provide them with content that is useful to newcomers to the topic. This means beginner-level guides (created by the community), frequently asked questions, opportunities to ask the most obvious questions, how to improve quickly etc.
If most of your newcomers are new to the idea of participating in a community, you need to educate them about communities. You need to teach them about socializing online, demonstrate simple, fun, interactive experiences. You need to satisfy their need for information through the prism of interaction. You need to help them build early relationships.
If your newcomers have participated in similar communities before, you need to educate about your specific community. That means community history, regular culture-orientated material, and information about top members in the community etc…
Of course, it wont be this easy. You might have 50% new to the topic, 30% new to the idea of a community, and 20% new to this community only. That means either performing a difficult juggling act – or catering to the one group you believe is most likely to become regulars.