1. There is no next step from 18-55 unless you really know what 18-55 is capable of, you fully understand the concept of lenses e.g. what lens to use in a specific condition, how focal lengths make a difference what is f3.5-5.6 and how it will differ from a f2.8. Did you notice you keep your lens at 18mm or close when you are shooting landscapes, and you are at 55mm or close when you are shooting wildlife? The 18mm is the wider end, and 55mm is the telephoto end. Conclusion is; for landscape we need a wider lens, and for wildlife we need a telephoto lens. To shoot a lion (with camera :P ) I am sure 55mm cannot get you close enough, so you might want a 200mm to get even closer?
Or may be you are shooting a wedding inside a church, and it's pretty dark there, an aperture of f1.8 will make shooting in dark conditions easier than shooting with an aperture f5.6.
So these little things that you need to study and understand and choose a lens for yourself, when you actually know every single aspect of lenses.
2. No, filter is not necessary. Most of the people (amateurs especially), have a filter attached to their lens, known as UV (Ultra Violet) filter. To be very frank it does not do anything, other than protecting front element of the lens.
Then their are specific types of filters, for each type of photography, but again they are not necessary. First you need to understand what each filter does. Just an example, if you are shooting in extreme bright sunny day, and you need to shoot a waterfall with that dreamy effect on water. For that effect you have to keep your shutter open for a longer period of time, e.g. 1 sec. And if you do that on a bright sunny day, the picture is going to turn out white and so bright that you won't be able to see anything. To solve this problem, there is a filter known as ND (Neutral Density) filter, which reduced the light intake of the camera, and let's you keep the shutter open for a longer period of time.
Another famous type of filter is polarizer filter, may be you can Google it and see, what it does (consider it an assignment :P ).
3. I did not understand this question at all. Live view is the view of scene when you see it live on the LCD of your camera. What do you mean bu there is nothing on your screen, do you not see the scene on your LCD? please explain, and I might be able to answer, or just message me.
4. Best photos of an aquarium: Hmm... well... it depends if the aquarium is placed in sunlight, indoors, dark place, studio...
Well, let's put it this way, this question is like, at what speed shall I drive my car in NY? .. may be 60mph? so what if there is traffic congestion, or a traffic jam or children passing by? So you have to drive according to the situation, there is no rule about it. Same is with the camera, there is no rule about it, you have to understand the trio of ISO, shutter and aperture and then adjust them according to situation and requirements.
5. Lens hood is not necessary, and is recommended only when shooting in direct sunlight or harsh lighting (stadium lights e.g).
6. I suggest you go to flickr, and search for images of Nikon d3000 and then Canon rebels, you'll see those shot by pros with a 5 years old 100$ dSLR are sharper than those shot by amateurs with the latest Nikon d3x.
So, the same old answer! it doesn't matter, there is no difference between Nikon d3000 and Canon rebel. The difference is the eye behind it.
Hope this helps! keep learning and good luck!