When you get objects like a tv or stereo to fix for another Sim, they get put in the family inventory, which can be found in Buy mode and then look for the box. - Click images for larger view.
To take them out of the family inventory, you grab them with the hand tool and drag them to a spot that your Sim can work on them.
I tried to get a photo of me grabbing a tv, with the grab hand included, but it It wasn't showing the grab tool. When you have things grabbed, they will be highlighted.
To cook something for someone. I've not had the best of luck with this one. I ended up having to use perfect quality ingredients for it to count the opportunity.
Create the requested dish, and place it into the Sim's personal inventory which can be found in the Sim Panel (Look for a backpack icon)
After you've done that, and it's time to take the dish to said Sim, a notice should pop up above the Sim panel to let you know it's time to take the dish to the Sim. You have to click that so your Sim will take the dish to the other Sim. - Keep a watch out over your Sim, as sometimes your Sim doesn't follow through with the opportunity.
For example in my game, Jared has to go to the bookstore to finish his opportunity, but it's not time. When it's time, it will be highlighted.