Using good lighting is one of the most effective ways of making your production look a million (perhaps even a trillion) times better, and having a kit ready makes it all a little easier and efficient.
If you can afford to put together a professional lighting kit that would make even the most pious sun deity hide their face in shame, then you're in the minority. The rest of us have to beg, borrow, and steal (don't) to get our films lit, but you know what -- sometimes you just need to get real guerrilla with it and use everyday items and DIY solutions. Our buddies over at Film Riot have come up with tons of ideas on how to build your very own lighting kit out of stuff you can find at any hardware store or even in your very own house.
Film Riot has helped so many filmmakers save money on production by showing them cheap DIY workarounds to popular filmmaking items and techniques, like how to make DIY ringlights and light bars, alternatives to gels, and cheap diffusion solutions. Putting all of these great ideas together to build your very own lighting kit will save you so much money on rental costs, not to mention the time that you'd spend fussing with rental houses.
Having my own DIY lighting kit has proven to be so much less of a headache than going through the rental process. Just get a bunch of clamp lights, some cheap diffusion material (sheets, shower curtains, etc.) and DIY bounce cards and you're already so much better off than you were without them. Just remember that using lights doesn't guarantee that your final product is going to look good, because it's not about using lights, it's about using light. Learning how to shape it and control it to get the images you want is a key part in telling the stories you want to tell.
If you don't know where to start, why not learn four basic concepts of light to understand the way cinematographers think about it.