What I will do tonight is something that actually will happen. What I would do tonight means what I want to happen but something is preventing it from happening. examples: I will watch TV. This is going to happen. I would watch TV if I had electricity. This is not going to happen because without electricity, the TV does not work.

When I say I will do something, that means I will likely to do it for real in the future (90%+) (＝つもり） When I say I would do something, that means I will probably do it, or maybe not.　（＝かもしれない） In other words, you use "would" when the activity you think about is merely one of what you want to do in the future, you may or may not do it, depending on what awaits you in the future. But when you use "will", you imply that the possibility of doing that activity is really high. Another difference between these two is when you use "use", you will sound politer, Just consider it 敬語 in Japanese. And of course, can't rule out the fact that "would" is also "will"'s past form can I, it's just too obvious, I suppose I need not talk about it. However, I don't think you need to think about it too much, no one really cares about what you use in real conversations anyway, as long as what you say is understandable. Hope that helps :).

"What will you do?　今晩何をしいますか？ "What would you do?" 選択があったら、何をしますか？ The first question is a question asking a specific question. The second question is asking a hypothetical question. I hope that helps!! :]