Hi,

how could i do it?

i would like that visitors could read posts in the forum without registering to site.

yes first available module for anon. users
then in forum admin --> edit forum --> Access Level :registerd users only

irmtfan is right on.

After installing the newbb module in the modules admin, the you need to make it available to the anonymous group of users by doing:

Admin -> Groups -> Anonymous users -> and check box the newbb forum module to give them access to read messages.

At this point anonymous can post as well, so just like irmtfan said you need to:

forum admin --> edit forum --> Access Level :registerd users only

Then boom. Anonymous can access/read forum messages, but has to register as a user to participate in the forum discussions.

I hope that helps./