For kids and amateurs: write every day, even if it is only a little bit.
Read the greats. Use libraries and books stores as well as using your school books.
Take classes. Those in school will be provided with English lessons. Those in college and university (long story) may have to choose to continue to get English lessons but now there is more free choice about literature and writing classes than before.
Writing workshops. These can be taken as one offs. Many are shorter than entire semester long courses and do not require full university matriculation. There are various types of teachers of writing work shops and it makes sense to ask them if the work shop curriculum is not clear.
Writer’s groups: taken with a grain of salt, but true.