How can I buy more storage space for my phone please help me step by step to do that.I’m staying in South Africa

You might want to take a look at this explanation:

What's the difference between device storage and iCloud storage? - Apple Support (GW)

Hello Somgatsha,

You cannot buy storage for your iPhone, but you can buy storage for your iCloud.

Best wishes,

Jay

I want to buy storage for my iPhone se, but it’s old storage is 16 and I want to be like 32 or 64 so how are my going to by that

You can’t add any more storage space to any phone. If you need more space, then you will need to delete something or get a new phone.