I hope you did well with your graph. I'm entering the following explanation for those who might still benefit from your question:
Your graph will look like a large L shape. As you go up the left side, put higher dollar amounts to measure as your monthly cost increases from the 20 dollar minimum. Your number of minutes will start at zero at bottom left and will increase as you go from left to right across the bottom.
What increments should you use to label your dollar amounts for cost going vertically on the left and the number of minutes across the bottom? If you figure that your phone bill is monthly and that you'll use your phone for a minimum of about 500 minutes to 1000 minutes or so per month (just to use a range of round numbers in terms of your monthly minutes), then use increments of 100 for minutes across the bottom. Starting at about 400 and increase by 100 all the way to 1200 to 1500, depending on how many monthly minutes you want to measure. Use the 20 dollars minimum for your cost at the bottom left of your graph. Use your formula: C = 0.05m + 20 to determine about how many dollars you will need in your range on your graph: 0.05(500 minutes) + 20 = $25 + $20 = $45. 0.05(1500 minutes) + 20 = $75 + $20 = $95. This means that you could label your dollar increments (up the left side of graph) as follows: $20, $30; $40; $50; $60; $70; $80; $90; $100. Label your minutes across the bottom starting at the left as follows: 400 minutes; 600; 800; 1000; 1200; 1400; 1600. To graph the first example I gave above, graph the 500 minutes by finding (across the bottom where it is half way between the 400 and the 600) and mark that spot at the bottom. Then graph the resulting dollar amount from where we used the figure of 500 minutes that we plugged into your formula/equation: 0.05(500 minutes) + 20 = $25 + $20 = $45. Starting from the bottom left side, go up until you are half way between the $40 and $50 to mark the spot for $45. Draw a very light or invisible or dotted line straight across from the $45 spot until you come the spot that is directly (straight) above where you marked the spot for the 500 minutes. If you want/need to construct a vertical bar graph, for example, then draw your bar upward from the spot where you marked it for 500 minutes and stop when you reach up to the spot that corresponds to the $45 that you marked on the left. In this instance, make sure that the width of the bar is centered on the spot that marks the 500 minutes.
It is very important to know whether your instructor or teacher has any specific requirements as to what type of graph (bar graph, line graph, or other type of graph) or any other specific guidelines or instructions (such as whether it is to be a bar graph that must be vertical, or pencil, or specific colors in marker) that pertains to your particular assignment for constructing your graph.
Have a great day and a great summer.