Start with an open hand, with your index finger touching your thumb, as if daintily holding a tea cup. Hold your index finger and thumb by the side of your cheek, close to your lips, while moving the rest of your hand backwards and forwards.
The sign for fruit is based on the ASL sign for the letter 'F'.
Wait until baby is a little older before teaching collective words like fruit. In the early days, I will instead use more specific words for the exact fruit I am referring to, like apple or banana. Babies find it easier to conceptualize the specific. However, it is cognitively valuable to eventually start giving them examples using a collective word, like fruit, which can refer to lots of different types of objects.
When we teach the sign for fruit, we do it by creating groupings of fruit and groupings of vegetables to aid with contrast.