The instaneous velocity of the object described in the graph at a partycular time t, is simply it's velcoity at that time. In other words you don't have to do any calculus (like you weould have to do if you wished to find the average vellcoity over a certain time) just read the value from the graph.
Are you sure this is what you meant? As jcsd pointed out, if you really have a "velocity- time" graph, then the instantaneous velocity is just the "y" value of the graph at time t. If you have a "position-time" graph, then the velocity is the slope of the tangent line at the point. If you have a "velocity-time" graph, then the acceleration is the slope of the tangent line.
If you only have the graph find the gradient of the grpah at that particular point, if you have the equation differentiate v with respect to t.
As I said before: if you have a "Velocity-time" graph, draw a tangent to the curve at the given value of t and find its slope.