How big is the panel? What power is it rated for? In full sun, disconnected from the battery, what voltage does it produce? Measure it with your multimeter. (Don't say "I haven't got a multimeter", you know what the reply will be!)
Then work out what resistor values will reduce that voltage to below 5V when used as a voltage divider. Aim for the total of the two resistors to be around 10K to 20K. Post your calculations and we will check them for you.
The problem with WiFi is that it is not very low power, for battery powered circuits. If you want the Arduino to respond to web page requests at any time, so you can view the charge level, a WiFi enabled Arduino is going to consume around 80mA continuously. That could be more than your panel produces on less sunny days, certainly at night! One strategy is to have the Arduino spend most of its time in deep sleep, to reduce power consumption. It could wake every 15 mins, read the charge level and transmit this to a web server on your local network or on the internet. You can then view the charge level at any time. But this all makes the project more complex, so think very carefully, especially if this is one of your first projects.
If you are sure it must be WiFi, then I would recommend not using the Mega. Get an esp8266 based board such as Wemos Mini and use that instead of the Mega. This makes things considerably simpler. Note that a Wemos cannot be powered by 12V directly, so a dc-dc converter is a good idea, adjusted to output 3.3V that the Wemos uses. This is much more efficient than a voltage regulator.