Look for a mask that has N95 designator on it. A regular hardware store mask will not protect you. There are many different types of masks but the one that you will need has the N95 on it. Many of them also have N95 printed right on the mask.
When you put the mask on have the top rubber band go just above your ears and the bottom rubber band below. Adjust the rubber bands by tightening or loosening them so that it is a firm fit around your face.
Press the nose bridge to check that you have a tight fit and then check all the way around to make sure there are no gaps. If there are any gaps then you need to readjust the mask by either tightening the rubber bands or shifting the mask on your face.
All these steps will assure that you will be protected. Check your mask periodically throughout the day to make sure that it is still fitting right. If it becomes loose or doesn't fit as well, then put on a new mask.