Could anyone tell me what a grand piano is like?
I have a upright piano, but i am now grade 4 and i would love a grand piano.
I am only 13, so i can probably only afford a 2nd hand piano.

I am not really sure what to do.
Please can anyone help me?

I think you are unclear or mis-directed about pianos. A good grand piano is very similar to a good upright piano. And depending on the manufacturer may use the same harp and actions.

As you advance as a piano player I would encourage you to investigate and learn about the action of the piano and not so much the size and shape of the piano. Grand Pianos often are thought to be better than uprights. And although this can be true, the quality is in the making of the piano. As an experienced rebuilder I can assure you more care tends to be given to grand pianos, and many upright pianos are abused. But this is not the pianos fault, it is the owners.

I have seen lots of so-so grand pianos. People think they are great because they are a grand piano. And they spend lots of money having them fixed up or rebuilt, but honestly they were only so-so. And I have seen some fantastic upright pianos that would put most of those grands to shame. Both in their feel and response, and in their sound quality.

It is nice to have a grand piano, yes. But if you don't have the right sized room you are not doing yourself much good. Imagine a 10,000 pipe organ made by Schliker in your 1,000 square foot home, or a 450 pipe Schlicker Organ in your home. Yes, the 10,000 pipe organ is awesome but only in its proper setting.

Consider "re-considering" your thoughts about "2nd hand" instruments. I have many friends who have purchased 'new' instruments. And everyone thinks mine are better. I have only ever purchased one new instrument and it sits in a closet at my mothers house.

What I think you should do is relax and enjoy the music you are making. If the piano you are playing is meeting your expectations find out why. Have a good piano technician look at the piano it might only need the action adjusted (not cheap but less than a new piano). This is part of piano maintenance. If he is a good technician and you express to him what you are looking for he can keep his eyes open for one that might fit your needs. A good technician can give a sound board a single thump and tell if it has a crack, or if the bearing is just off. Is the pin block tight enough?

Here is what I tell my students, family, and freinds when they want a new instrument - especially when they want a certain specific brand or style. I tell them they are not ready for it. I tell them they will know when they are ready for it when they realize they don't feel they need it.
(sorry for the little lecture)