Tape measure, pencil, screws + rawlplugs (and appropriate sized masonry drill) or nails and picture hanging hooks, and a spirit level.
First, select the wall you think would suit the picture/s best. Using your tape measure find the middle of the wall section you want to hang the picture on, but allowing for things like open doors that may shift the central point. Place a small pencil mark at this place at approximately eye level (somewhere around the 1.6 meter level). This should be the center of the picture, so next measure the height of your artwork. So, if, say your picture frame measures 40cm, then, using your tape, place a small pencil mark 20cm above the center mark. Next, looking at the back of your picture, use the foot of your tape measure to pull up on the mid-point of the cord or wire of the frame, and when it has been pulled up tight, measure the distance to the top of the frame. This is the distance you need to measure down from your top pencil mark, to find the exact spot to place your screw or nail. If you are going to hang two or more picture in a line, then use your spirit level and tape to mark the appropriate spots.
Bear in mind other features that could affect your decision, such as furniture under the hanging point that may require you to raise the artwork higher, for example, or even if it just doesn’t look right in the measured center. Remember, the above is only a guide, and some situations call for judgement by eye, rather than tape measure. What actually looks best, is what is best!