Could be a number of things. What to check:

- Any sharp burrs in the rim, and that the rim tape is seated and no spoke holes are exposed.
- Any pieces of glass, tacks, etc in the tire itself. Don't just inspect the outside of the tire, but the inside walls as well.
- That the tube is seated properly and not underneath the tire bead before inflating. Once you've seated the tire onto the rim, it's helpful to inflate it a little bit, ~20psi, and rock the tire back & forth on the rim to release any of the tube from underneath the rim before fully inflating.
- Don't use the lock ring that goes on the valve when inflating. Sometimes this pinches the stem of the valve.
- The tube is recommended for that tire. And likewise, that your tire is recommended for your rim.