In case you have a lot of new users register on forums that simply post spam or their are promote their own websites. This tutorial will give you instructions on how to moderate new users until they receive a certain number of posts.

1) Create a new group for your approved members.

Admin CP -> Usergroups -> Add New Usergroup

At the top it says "Create Usergroup Based off of Usergroup" in the dropdown choose Registered Usergroup and then click Go if necessary.

In your new usergroup, you can change the name and description if you want.

Scroll to the bottom and just above the save it will say "Create Permissions Based off of Usergroup". Choose Registered here. If you have no custom permissions for the registered usergroup or it says none, just click the save button.

2) Setup a new Usergroup Promotion.

Admin CP -> Usergroups -> Promotions

Use these settings, where X is the number of approved posts that are required:

Usergroup - Registered Users
Reputation Comparison Type - Greater or Equal to
Reputation Level - 0
Days Registered - 0
Posts - X
Promotion Strategy - Posts
Promotion Type - Primary Usergroup
Move User to Usergroup - the group you created for approved members

3) Run the scheduled task for Usergroup Promotions.

Admin CP -> Scheduled Tasks -> Scheduled Task Manager

Click the "Run Now" button to the right of the Usergroup Promotions task.
Finally remove you need to adjust the permissions of the Registered Usergroup so their posts and threads are moderated. Go to the Usergroup Manager and edit the registered usergroup.

Admin CP -> Usergroup Manager -> Registered Usergroup -> Edit Usergroup -> Go.

Set "Follow Forum Moderations Rules" to No.

Press Alt-S or click the SAVE button at the bottom of the form.

You will need to check the moderation queue in your UserCP on a regular basis to approve or delete messages from new users.