If I have 0$ at the moment can I make 100$ in 2 or 3 months? How?
What if I deposited 20$ .. Can I make them 100$ in less time?
Well there are a lot of ways to earn off this forum.
The most common way in your instance should be to join a signature campaign and essentially slave labour for the big bitcoin companies to advertise their sites, lol. But shop around, you can get usually better offers just by shopping around for like 10 minutes and looking at the terms and conditions of every campaign on offer right now.
If you had $20 it really doesnt help because thats just such a small amount you cant do anything with it, you cant invest it, you cant trade with it.
If you want, you could gamble that $20, although youll probabaly lose.