I'd recommend an ESP32 (or it's slightly less capable little brother the ESP-8266) using the Arduino environment to get started. If you prefer Javascript, you can program it that way using something like NodeMCU or MongooseOS. Once you get comfortable with that, if you want to get a little lower level you can use PlatformIO with the vendor's SDK directly.

I like the ESP boards from Wemos. The US made ones from Sparkfun and Adafruit are also really good. The advantage of the Wemos ones (and the many counterfeits you'll see on sites like Aliexpress) is that they're so cheap you don't even have to feel bad if you fry one.