Ranieri was announced the new Fulham manager this week, replacing Slavisa Jokanovic, with many celebrating the popular Italian's return to the Premier League.
"If I came here it's because I believe there is enough quality here," he told 'Sky Sports'. "I am mad, but not stupid.
Fulham are bottom of the Premier League table after twelve games.
"When I went to Parma, all my friends said you are mad to go there. When I was at Cagliari, they were bottom and everyone said we were out - I love this kind of battle. It's my character.
"My friends didn't say I was mad this time. They said 'good choice, come on, you can do this'. I think I can do this. It's not easy but I believe it.
"I don't know how much time I need to improve the defence. It depends how long it takes them to understand my philosophy.
"We have to defend altogether and attack altogether. I have to work hard and I know I don't have time. It's a big battle.
"When changing the manager, you change the air in the dressing room. Maybe some players didn't play with the previous manager and with the new manager they give more, because maybe there is more feeling.
"I believe I have very good players but they must have to show me the fighting spirit."