Lead the 8H. Declarer wins and returns a diamond, go up with your AD and then lead the TD. If you make these plays the contract will go 1 light.
Comment
Usually second i hand you would play your lowest card, but with only 2 diamond cards, you want to unblock the diamonds so that if partner has subsequent winners in diamonds he can play them.
The above commentary is intended as a guideline only. As with any Bridge hand there are many ways to bid and play any hand.