you will need three : one for applying your e/s, a blending brush, and a small brush, angled or not, for eyeliner or small lines.
i have a blending brush from sephora, and i'm very fond of it. you can start by sephora brushes, but if you have some money to spend, the mac brushes are worth the money. if you take care of them they'll last long.
here's pics from the mac website so yo ucan see what brushes i'm talking about :

(Mac 213)

(Mac 224)

(Mac 266)
or :

(Mac 209)
this one is also helpful for inner corners :

(Mac 219)
those are only examples, of course, and maybe you'll need more later, but the first three are brushes i find useful each time i use e/s, which means almost everyday !