Your eyepieces are the first accessories you should have and use with your telescope. Since they are interchangeable, a telescope can be used at a variety of powers.
Look through the eyepiece by placing your eye just behind it to take advantage of its eye relief, the spacing between the lens and your eye. This should be at least 15mm for the best comfort, maybe more if you wear eyeglasses. You’ll lose field of view if you place your eye farther away and may even move your eye out of the beam of light from the eyepiece. Getting too close will prevent you from blinking and also cause a black ring to appear around the field of view.
Now turn one of the two knobs to the side or below the drawtube, first one way then the other until the object is in focus.
Always start with the lowest power eyepiece (the one with the highest number in millimeters printed on it). It is much easier to focus and has a wider field of view making it easier to aim the telescope.
As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations.
Most of your observing will be done with lower powers. With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.
A good way to increase magnification is to use a Barlow lens. A Barlow lens rated at 2x can be used with your existing eyepiece and it will double the magnification of any existing eyepiece. Since longer-focal-length eyepieces generally have longer eye relief, using a Barlow to increase magnification will allow more comfortable high-power viewing.
A basic set of eyepieces would be 40mm, 25mm, 9mm and a Barlow.