Hey there, it's a good thing to be proactive

Nice to hear you want to start on stuff now - will give you plenty of time to get your work formatted!
Even if you're not sure where you want to apply yet, I would have a good look at what architecture and interior design courses often ask for in their portfolio guidance. There's some great resources out there and some examples of people who have submitted successful portfolios for these courses. Try doing some googling for "Architecture portfolio examples" etc. It might give you some benchmarks of what to aim for.
Some generalist advice - it's a good idea to have examples of
iteration/development/design work (often the kind of thing you might do in A level Art & design), some
drawings from life (e.g. still life, figure drawing, technical drawing) and
imaginative work (work that demonstrates combining ideas imaginatively).
Some places might request a variety of mediums, and these might be specific to the course you're applying for.
In your specific case I would expect to include
more technical drawing and still life drawing - observational drawings of buildings and interiors, focusing on getting your perspective as right as possible. Specific technical drawing - being able to draw
multiple angles and perspectives of an object/building/thing with a
good degree of accuracy - will also likely be particularly valuable to you. This demonstrates an ability to think in 3D space - as does
model making, prop making, 3D mediums like woodwork and metalwork or digital 3D visualisation and modelling (i.e. with SketchUp), which I would also include examples of if it's possible for you.