First of all, one thing we are sure of is that as time passes the Sun is getting hotter and hotter. The best estimate at present is that all life on Earth will cease in about 2.8 billion years because by then the sun will have become so hot that not even the simplest life forms (of any kind that we know of, anyway) will be able to survive here - although plants and other more complex forms will have gone long before that.
As for the Sun itself, it is supposed that it will continue to heat for over a billion more years after that. The sun probably has enough fuel for another 4 or 5 billion years from now. When it finally runs out of fuel it will become a red giant (an inflated star at a late stage, with an inflated outer atmosphere) and finally explode. Some scientists think that a smaller, cooler white dwarf star will be left in its place but others don't agree - you can learn more about the different possibilities here.