Okay, first comes first
It doesn't look like a disaster to me and you can for sure fix this one.

If it comes to the brushes some easily break even if right out of the package while others last for years.

1. I believe you should use a sponge or cloth to wipe the colour over the entire canvas This will go quick and you can let it rest till the next day.

2. You can paint with with nearly everything. Try out what feels best for you. Again it can be a sponge to dot the clouds, shape the mountains and dot with the sponge on different parts of the mountain (safe the pencils for details or let it be)

3. water is the same as the sky and mountains, just smear the paint on the canvast . You can use different layers. Let it rest till you know what you will do next.
   Keep in mind that the great old masters rarely made a painting within one day, it was more like a year or even years and in many cases they weren't the only ones working on it, their students did too. The Mona Lisa for example is never finished.

4. It will be very hard to make a lighter colour. If you mix paint it goes from light to dark. You take a light colour and ad a tiny bit of dark to it (unless you will need a lot of paint.

5. If you want to make light what is already dark on your canvash you need layers and "wash" it... so a wet brush/cloth/sponge and a tiny bit of a light colour (white/yellow, light blue) and add that layer and see what happens. You can practice

6. It would be could to have a board you can place on the easel and tape a big paper on so you can try out different ways of working. If you have cardboard (like from a box) you can use that too to practice with the colours, brushes and so on.

@almaguer his first paintings were landscapes too. I hope he can give you some tips and show you what he used for an example and technique.

So far so good, Rome is not built over night and what we have in our mind is rarely what we create (and least not with me).

Don't give up, and let me know if you need something.