How to send Unassinged Submission for review

I am using OJS 3.0.2. The new submissions are categorized under Unassigned table. Can anybody tell me why these submissions are unassigned? Further, how to to assign these submissions for review process?

Thanking you in advance.

I suggest you read the Learning OJS3 guide. It explains the whole process from the editors, authors and reviewers point of view: Editorial Workflow

Also I recommend that you upgrade to The 3.0.2 is fairly old and has a lot of small bugs.