Move your cursor around the golf ball to aim, hold down the left mouse button to start the power meter, and click once it's reached the desired power. You'll be able to aim for your next shot as soon as the golf ball has stopped moving.
BasketGolf is, as you guessed, a cross between basketball and golf. But maybe not as you'd expect - it's more of a golfing game than anything, and the only element of basketball that this game contains is the fact that some holes resemble taking a basketball shot - but that's it. For all intents and purposes, BasketGolf is a golfing game - so play it like one.
You'll begin each level opposite of the goal. It's up to you to make shots that will land the ball into the goal - sometimes this can be pretty straightforward, while other times you'll have to conquer certain obstacles one at a time. This can lead to some tedious levels, so if you don't have a fair amount of patience when taking your shots, you may be in trouble.
But if you take your time while aiming shots, you'll be alright. Actually, when measuring the power of your shot, the power meter (the blue arrow) will fill up and deplete over and over again, so don't think you have to decide on the power immediately. If anything, this means you can make a better shot, so take your time.
Overall, BasketGolf is simply fun to play, if not a little aggravating in later holes - you'll have to deal with obstacles as soon as you reach them, and sometimes getting a hole-in-one isn't possible in this basketball/golf hybrid. If you take your time when making your shots, though, and try not to do too much in each shot, you'll do fine. Aim for the lowest amount of strokes possible to get a highscore.