I'm really confused why do you have SELECT-OPTION and then change it in START-OF-SELECTION event. This doesn't seem to make a lot of sense... Select options are meant for users to enter their, well, selection criteria. If someone enters one thing and then the program plays "switcheroo" on them wouldn't that be confusing?
If you need to pre-fill some values for the users (i.e. defaults) then it needs to be done in INITIALIZATION. Or if you just need to build a range to use in SELECT somewhere then take a look at Help article for 'range'. Or maybe you don't even need anything and could just use SELECT ... JOIN. This example looks very confusing to be honest.
And if you're selecting just one field then you don't need INTO CORRESPONDING FIELDS.
Internal tables are similar to arrays in other languages. Not exactly the same but close enough.
I'm also not sure why would anyone want to learn ABAP at this point since it's clearly a dying breed but that's another story. 