Here are answers to your questions
1. You'll only get a response if you ask for an autographed photo. If you write only a letter, then you will get no response, not that I know of. If you ask for an autograph and send a letter, however, you might get a bit more than an autograph. I wrote to Dawn Wells, who played Mary Ann in Gilligan's Island, telling her I think she played Mary Ann wonderfully and that I'd like her autograph. Not only did I get her autograph, but I got my letter back with the words, "What a nice letter!" on the back of it.
2.
Always write a letter! First of all, you should let them know you want an autograph. Don't send just a picture and envelope! The letter is called an LoR, letter of request. And second, but just as important, give him/her compliments. Tell them about what you like about them, not just the character they played or that you like the words of a song. Write how amazing his/her talent is! That makes you more likely to get a response.
3. Look at feedback that other people have given, and you should know the proper time you should receive a response. I looked at feedback for Christopher Plummer (Captain von Trapp from The Sound of Music) and saw it takes him from one-and-a-half months to three months to respond, so I waited that amount of time. Sure enough, it took one-and-a-half months! It should happen at the same rate others received autographs, but it could also happen sooner or later. Just have patience!
However, there's an alternate answer. I was getting worried when I hadn't gotten a reply from Kathryn Beaumont (Wendy from Peter Pan and Alice from Alice in Wonderland) still hadn't responded. She seemed to take three months, but it had been six months, and I hadn't gotten anything! I then learned that celebrities are given batches of autographs at a time. Everyone had responses with her at once but had sent letters at different times, so there was no pattern. Just have patience! It's hard, but it may eventually end up in your mailbox!
4. If it's only a letter, just send a letter-sized envelope.
I hope this helps!
FCM
P.S. I suggest you change the subject title from "Hello people, Let's meet!" to something like "Autograph questions". The title you have sounds like spam and will likely be deleted.
I love hearing others' opinions! However, before you start commenting, please read this carefully:
I welcome opinions but will not obey orders and suggestions. So, please don't say, "Toss that in the trash," or, "Frame the autograph right away," etc. I understand that you're sharing what you'd do, but you can't make me do what you order--what I received in the mail is mine, not yours.
I've changed the topic. And the things you said are very helpful. But I want to ask one more question. What is SASE? I see that word in some feedbacks. And I couldn't understand 
Or is it better if I send it as fast as possible?