Bowing marks
Bowing marks up-bow 
(at the tip of the bow) and down-bow 
(at the tip of the bow) can be found in the trill menu.
With bowing marks, legato arcs and articulation signs largely determine how strings should play their part.
Under [Options|Music|Instrumentation] you can further specify how these characters should appear on paper in terms of color, size and font.
To prescribe 'plucking' with the fingers, a technique called pizzicato, notate pizz. or in full pizzicato above the part. To indicate that it is time use the bow again, write arco.
In sheet music for strings, legato is frequently used, essentially legato arcs that indicate which notes should be played one bow.