This is either a bladder snail or pond snail - you can tell by which way the shell curves (does the spiral open to the right, or the left?). I believe pond snails also have slightly thicker eye stalks. My guess based on the photo is bladder snail. They're a very very common pest snail, and can reproduce asexually quite rapidly.
Some people don't want them, and try to remove them - if you don't want more I'd suggest removing this one as soon as you can, before it can lay too many eggs, and look out for the clear jellylike egg clumps that might be in your new plants. However, others consider them to be beneficial in cleaning up waste on the substrate, and they shouldn't reproduce to excessive numbers unless you are overfeeding the tank.