Overhand knot

This is as simple as it gets. That said, it doesn't hurt to go over the basics so you feel comfortable learning knots of increasing difficulty. Start to get comfortable with the ropes and explore.
Overhand knot Shibari Academy

Square Knot

Pay attention when tying this knot. It looks simple, but it's easy to end up with a grandma's knot if tied incorrectly.
Square Knot Shibari

Square Friction

Great to attach ropes that are perpendicular to each other and then continue tying in such a way they are parallel to each other
square friction shibari