My car 99 Nissan Altima creaks/rattles in the front when going over bumpy road. Any reasons why?

Sounds to me like your front suspension has some worn out or dried out bushings. These are rubber sleeves in holes where the suspension mounting hardware pes through the mounting points. Jack up the car and look at each control arm mount, stabilizer bar, and shock/strut mounts. You are looking for cracked or missing rubber in the mounting joints. If there are no cracks, you can lubricate the bushings with Silicone spray. Don't use anything with oil or petroleum on rubber parts. If they are cracked or missing, they need to be replaced. It's not a huge amount of cash but can run a hundred or two. It's also possible that you've got a bad strut or two. Does the car bounce easy?