Which flower/plant to use?

I have this and I want to put a nice simple plant in it that has nice big white petals!



As you can tell I am an absolute beginner that has no clue how to go about gardening! I'm okay with this as I have to start somewhere so I thought this little project would be a good starter and hope that you can really help me understand how to go about this.

First off, my idea of nice big white petals is just that, my 'beginners' idea. I'm wanting to keep an overall simple contemporary look in the garden. I've just had the patio and lawn (already got help from you guys regarding that) done.

I don't know what is possible such as:

Do or can I buy a plant already grown from stores like B&Q or Homebase?

Can I plant only certain ones at this time of year where I'm already too late to plant some nicer ones?

Are there plants for this that flower all year round?

Will the vine (sorry, don't what's called) already planted in there suffer if another plant is put in there?

How do I actually plant or seed!?

I may have put too many questions in here so I'm not expecting them all to be answered but thought it would give you an idea what my thought process is like towards this!

So in short, if you could recommend which plant would like nice in there then I would really appreciate it.

In the meantime I will do a bit of research to get a better understanding of what's involved and possible.

Thanks you so much.