I am trying to define a function that draws a triangle of a given size using the turn method. I've never used turtle graphics before and am not sure how to go about this.
Thank you

In simple terms, to form the triangle you can draw 3 lines of the same length. To connect the lines, you turn the turtle by 360/3 = 120 degrees after each line has been drawn.

Example ...

import turtle as tu
tu.forward(200)
tu.right(120)
tu.forward(200)
tu.right(120)
tu.forward(200)
tu.right(120)
tu.done()

You can simplify this with a for loop. Also start out the turtle at some other point, default is the center of the canvas.

tu.done should be in main body of your code.
Use these functions: xcor and ycor
gives x and y positions of mouse goto(x,y)
going to given absolute cordinates

define triangle by giving only two sides and then recording starting point before starting, draw given side lengths and turn angle between them (for example)
def drawTriangle(x,turn,y):

Here is test for your main program

for i in range (0,360,20):
print
drawTriangle(250,250, 67) ## first side length, turn, second side length
tu.right(20)
tu.done()

If you want to draw a triangle knowing only it's three unequal sides, then things get a bit more complex. You have to apply the law of sines, which states that the ratio of the length of a side to the sine of its corresponding opposite angle is constant.