Hello everyone.

I have upload YAF forum to my server. my database also in the same server

What i need is to externally (not inside the forum). To create a query that will insert a user to my forum

what is the general query for such a thing.

I tried insert directly to yaf_user but it could't work

You should check the table yaf_usergroup and add each user to the Registered, Registered Users and Subscribers groups.

Check the following tables, to link proper IDs of users:
yaf_group (IDs of groups)
yaf_usergroup (this links each user ID with group IDs, so each normal user would have three rows).

(EDIT:
There might be more things to do, if you do NOT use YAF's DNN Module)