Press the arrow in the corner to open the shop, and buy turrets from there. Press the play button and watch as your turrets shoot down the enemies! Click on a turret to view it's upgrades and range. Note: If a turret is shooting out of it's range, that just means it's range is too big for the circle to comprehend.
Thank you to @warfame for the tutorial. A Heavy thank you to Heavy Tower Defense by @cs2889410 (you should check it out), both as an inspiration to do this and as I borrowed some of their sounds and textures, and when stuck used some of their code. If you see any enemies but they die so quickly you have no idea what they do, comment below and I'll try to let you know!