If Earth is positively charged, why is it so, is there any negative
charge around it which helps Earth retain its positive charge ?
It's actually a little more complicated than that.
The earth around you locally is part of a giant circuit!
Thunderstorms generate a negative current which then flows back to the ground wherever there aren't thunderstorms. So it really depends on the weather. It also depends on a variety of other factors like solar storms and ionization of the upper atmosphere, but this gives you an idea of what goes on.
I coudln't find the graph but the local electric field also changes when the sun shines from ionization. The electric field above also contributes charge, thunderstorms and other effects all contribute. All in all, the ground's net charge can be considered zero and a reference for all other charges.
Also, what happens to charges that flow to the ground ?
The ground also has conductivity (or functions like a resistor), so it distributes the charges (as in a lightning bolt) or the really low currents that come from the air to the ground to keep it's potential the same. There isn't really a good way of determining the earths total charge as you would have to account for all of the factors. Just call it 0V for now.