When to use it
"I have done X" — recent and relevant
Use the present perfect for events in a time period that includes the present — today, this week, this year, ever in your life — and for things that have just happened. The key word in English is "have / has".
Common companions are ya (already), todavía no (not yet), alguna vez (ever), nunca (never) and recientemente (recently). In Spain it's used heavily; in Latin America the preterite often does the same job, but for GCSE you should be confident with both.
Model sentence
Esta semana he estudiado mucho para los exámenes.
This week I have studied a lot for the exams.