You can download a pre-made Bluetooth terminal app on your android phone..
At least for initial testing purpose.. and just to get some sort of serial data into your Arduino.
Then you can focus on the parsing and execution aspects of the code.
From there.. you can circle back and make your own app.. I like MIT App Inventor 2 because its fairly easy to use.. and a beginner can make some basic apps with-in an hour or so..
In ther past I created an Android app that had some sliders on it.. one for R, G, & B channels.
I sent this 'pwm' value from the sliders/app to my Arduino.. and it parse the incoming data and applied the value to the correct PWM pin/color channel on the connected RGB LED
You could also use an ESP8266 wifi module.. configured as a captive portal.
Host your OWN HTML/JS/CSS webpage on it... and have it execute/parse on page submit whatever data you want.. (same approach as above once the page is submitted and parsed.. you then send whatever the web page submission action was over to the Arduino via serial from the ESP.
(I think of these type of projects as a 'local' GUI connected to my projects)...
I can connect to the broadcasted SSID (wifi network) with my phone.
I can open my browser (captive portal means any page I request will ONLY server up my internally hosted page that is on the ESP module)..
I click some links/buttons ...etc
The page submits (to itself).. the POST data (or whaterver) is read by the ESP and passed along to the connected Arudidno to control the connected assets/components..