Here's a couple of comments that would help you out in achieving "sharp" photos.
All lenses perform best when stopped down. The general rule seems to be 2-stops down from the maximum aperture, though this varies from lens to lens. In your case, the 70-300mm has a maximum aperture of f/5.6. You'll get better if you stop down to f/8 and f/11.
All the sharp photos you see online have been processed to a certain degree. For a quick and simple fix, apply an unsharp mask in your favourite image editing package. As a starting point I would go for an amount of 0.5, a radius of 5 pixels and a threshold of 0. This a technique called local contrast enhancement and helps make images look sharp. A more extreme form of processing would be to do the Dragan effect. This is considerably more involved, so I would suggest Google
Other things worth mentioning...
Your photo is over exposed. I'd underexpose it by at least 1 stop. I notice you've said you're using the manual mode. Why? I'd recommend putting it into "P" and then focus your effort into making good pictures (that means composition, lighting, post processing, etc). I do not mean to insult or discourage you in any shape or form, but stick to stick to the "P". That's what a lot of professionals do anyway and this mode is jokingly referred to a "P for Professional"
You've taken a shot of the tiger in direct sunlight. This is going to be tricky at best. You notice how the whites on his front leg have gone white and there's no detail there? Also note the bright rock at the bottom right of the frame. For best results, take a photo of them when they're in the shade. You get more even lighting which will make your subject look better.