How to get warranty for your product if you buy a gadget online?

A lot of products are sold online especially the gadgets like ipad, iphone, cameras, laptops, etc.
but do you have warranty for those products? What if something happens with your gadget after 1 month from the date of purchase?

Do they offer warranty for their products online? Most of them are not from the company itself, they are just selling brand new products online.

Is it safe to buy an Iphone 4 screen replacement kit off amazon.com?

A couple weeks ago i dropped my iphone 4 on pavement and the front screen shattered but I had a screen protector on it so NO glass fell out. I went into apple to see if they could just replace the front but they said it would cost 200$ to fix because they would give me a whole new phone. I bought the Iphone 4 through verizon. I was wondering if it would be cheapest to buy a screen replacement kit off of amazon.com and replace it myself. Any comments? Should I do it? And don't say to just buy a new iphone because i'm 15 and don't have the money

How to turn off cell service for my iPhone?

I'm going on vacation to Europe this Thursday. And I wanted to bring my iPhone to take pictures because my iPhone has a good camera. (I don't have a camera of my own and I don't want to waste money buying one because honestly I would only use it for this trip and never again). But the thing is I also just want a break from technology, I don't want texts from twitter or people so how can I turn off the service for the 3 weeks while I'm gone? Should I put it on airplane mode? Would that be okay because I'm techinically not on an airplane? I just really want no distractions I spend to much time on my phone at home, haha!