Click Add Relations 
(Dimensions/Relations toolbar) or to create geometric relations (such as tangent or perpendicular) between sketch entities, or between sketch entities and planes, axes, edges, or vertices.
You can also add relations while you create individual sketch entities, or when you select two existing entities. Existing Relations and Add Relations sections appear in all sketch entity PropertyManagers.
When you create relations, at least one of the items must be a sketch entity. Other items can be either a sketch entity or an edge, face, vertex, origin, plane, axis, or curve from another sketch that forms a line or arc when projected on the sketch plane.
You can also automatically create geometric relations with inferencing.