Here, several options are available to you. Indeed, you should know that the 2 leading operating systems on the smartphone market are iOS (Apple) and Android (Samsung, Huawei, etc.). To develop applications on these platforms, there are several solutions:
Which programming language to learn to design software?
If you want to get into software development, there are many options available to you:
Java, which is and remains a benchmark in the world of software development;
You can also learn C, one of the oldest and still in use languages;
Also aim for C ++, created to compensate for the lack of certain features of C (mainly the use of classes and all that follows);
Or even C #, also based on C, especially used in the Microsoft environment.