1. I can give you a hand if you want.

-That would be great.

(What is the meaning of 'would' in the answer?)

- That will be great.
- That's great.

(Can we use these answers? What is the difference in meaning?)

2. I can give you a hand if you want to.
-That would be great.

(Do we have to use 'to' after 'want' or not?)

1. It's better to use "That would" or "That's" rather than "That will" -- the reason is because "would" indicates a possibility, not a fact or a future fact.

2. You don't need to use "to" in that sentence.