How to make connection string direct to database that in project folder and open in any computer

You can't, not with SQL databases - you have to connect to those via an instance of SQL server.

You could change your code to use a single user database such as SqlCe, SqLite or even Access, but any system which wants multiuser access to the same database really does need a server system. (You can do it with Access - just - but it generally causes a world of pain at some point)