Eraser and ruler are needless. At the beginning of your drawing journey, you need to do everything by your hand. If the line is not straight enough, make a new try. Your hands will manage this over time, believe me.
Take care of the correct lighting. With paper, you should find a place near a window or sit next to a fluorescent lamp. Proper lighting is crucial for you to see all the shades and colors clearly. Read more on UpskillsTutor