Let $\displaystyle \mathbb{F} $ be any field. Let $\displaystyle K_n $ be the set of all $\displaystyle n \times n $ matrices $\displaystyle A $ with entries from $\displaystyle \mathbb{F} $ satisfying $\displaystyle \sum_{i=1}^n a_{ij} = 0 $ for each $\displaystyle j $. How do I find a basis for $\displaystyle K_n $ and how do I find dim$\displaystyle (K_n) $ ?

I really need help understanding how to find a basis and dim for any question like this.