You will gain the most opportunities for acting outside of school: drama classes, drama youth programmes, dance classes etc. And definitely singing lessons.
In terms of academics, definitely choose your Drama and Music, if offered by your school. Also join the school drama club, mentoring scheme, dance club, poetry class etc. Maybe also take art as well, but I don’t think you need this as a GCSE qualification.
Always be on the lookout for opportunities to improve your portfolio: small stage productions at your local theatre, volunteering to act in you friend’s film project, helping at kids drama lessons etc.