I'm not gonna lie to you, sometimes 8th grade can suck. For most schools here in the U.S. it's the year right before you go to high school and a lot of stuff is going to change. Despite the fact that you think that your shyness comes from having moved from Europe I feel that it's normal for you to feel shy and insecure at the age I was a new kid to and I understand what you feel and even though I moved here five years ago I STILL feel like a new kid sometimes.
First off, I know this might not be what you wanna here but, who cares if people think your question is stupid. Trust me on this, if there is something you do not understand in class you need to raise your hand and tell your teacher and ask them questions, if you don't feel comfortable enough doing this yet, then approach your teacher after class or before or after school if you can and ask them then. I have found that sometimes it is easier to approach a teacher when you are alone with them. While asking a "dumb" question may embarass you right then and there because once again I'm not gonna lie to you people will most likely laugh. I think failing the class because you didn't have the courage to raise your hand and ask questions will make you feel worse. Trust me it has happened to me before in my physics class. My teacher will explain something and I won't understand he'll explain it again and I still won't understand so I raise my hand and my classmates laugh at me, but honest to God I don't care because the feeling I get of actually understanding something in that class is soooo much better. So now to answer your questions.
1) You wanna be social but not too social. If you talk too much in class your teachers will not like you! Well... unless it's debate class or something. Be respectful to your classmates that way they will most likely respect you in today's world respectfulness is something you will rarely find, your classmates will find this peculiar and will perhaps be impressed and talk to you more. Maybe do a little research find out what's popular. What your classmates like. I'm not saying to completely follow the crowd and to be unoriginal, you can still like whatever you like, just look into there interests and see if you pick up a new hobby and perhaps meet a new friend by it.
2) I kind of addressed this already, it's normal to feel unconfident at your age.I'm going to tell you the truth, this isn't something that I can teach you to do, you kind of learn this as you get older. There's a point you will reach in your life when you realize there are a lot of more important things than constantly worrying what people think of you. My best advice would be to ask your questions after class if you don't feel comfortable doing it in the middle of class. Your teacher will appreciate your courage of approaching him or her and asking questions and later on if you do this enough, he or she will have picked up your learning style and believe it or not they will rephrase what they teach on their own to fit your learning style. It happens, I've seen it.
3) The best thing to do, or at least that I do is laugh too. Yeah, it's true sometimes my questions aren't brilliant and yeah it may have been something the teacher just went over 20 times, but could he do it 21 times? Cause I still don't get it. It may be stupid and embarassing but at least I know I understand now so who cares? Trust me, people will respect you more for asking the questions you need to really ask.
My last few pieces of advice are to make a friend. Who cares if it's just one, and Sally may or whoever is in your class has 23? As long as it's one good friend it doesn't matter. Perhaps when you have one you can try for two? You may be suprised to find that some people are shy like you, but these people mostly hide it. You can do it too. The only way to becomes less insecure and shy is to overcome your fears. How do you do this? By confronting them, you go up to a large group of people and talk to them, after all the worse they could do is ignore you, and yeah while that might feel bad, imagine how it would feel if they bit your head off? Hahaha, only joking. My point is that there is nothing to be afraid of except continuing to be shy and reserved, you're gonna miss out on a lot of potential friends if you keep that up.
Take it from experience. I know it's hard best of luck on that. I know you can do it!