When you remove the old tube, make sure you note the orientation in the tire. Blow the tube up and locate the leak. Lay the tube on top of the tire and mark the spot of the leak. Search that spot on the tire till you find the cause of the leak. Most likely, there is either something poking thru the rim, or there is something embedded in your tire causing a puncture, or a rough spot in the rim strip. Fix the problem that caused the puncture before you place a tube back in. When you place the tube back in be very careful that you don't pinch the new tube against the rim or between a tire tool and the rim. I never use a tool to place the tire back on, just my hands for that specific reason. Once the tire and tube are back together, look all around the tire and rim and make sure it is evenly and completely seated in the bead and nothing is sticking out. Inflate partially and check the bead again to make sure it is seated. Follow those steps carefully and your problem should be cured.