About 4.5 billion years ago, out solar system was nothing but a cold dust cloud floating in outer space. The dust came from even older suns (stars) that had exploded after much of their fuel was used up. (That's pretty amazing to think, eh? All the matter that makes up our bodies was once at the heart of an exploding star!).

The force of gravity caused the dust cloud to come together into a tighter and tighter ball. It began to spin and heat up and eventually formed the planets and the Sun (our sun is much smaller than the older suns that had exploded earlier). Learn more about it at:
planet formation

That is an open-ended question to which the best answer I could give you is "nobody knows for sure". The prevailing theories feature the sun having a disc of spinning matter around it as it collapsed to form a star, and that spinning matter itself coalesced to form asteroids, comets, and other bodies, which subsequently agglomerated to become planets, and those planets with a mass about ten times that of the Earth had enough gravity to then trap an atmosphere massive enough to swell them up to gas-giant sizes (the mass of the atmosphere alone being sufficient to attract more).

There are a number of questions concerning why the planets in our solar system have the orbits that they do and the distribution of mass that they do (i.e. rocky planets inside, gas giants outside), as well as why all the planets have orbits so close to circular. These questions simply have not been answered definitively.