Obstacles
You're going to run into a few obstacles with your device pretty quickly. There are many more obstacles (language barrier, health, etc.), but other questions cover that pretty well. This is just about the device itself, and related incidents.
Battery life
Your phone will likely run out of battery in only a few hours of use. Unless you can charge it, your phone isn't going to be much use. You might be able to write down some basic schematics for building a generator and a map of ancient lodestone deposits, but even with that it will be a long time before you get a constant 5 volt supply to recharge your phone.
The best fix for that would be to bring a solar charger, a hand-crank charger, or a camp stove charger. The stove would be best, then the hand crank, and finally solar power.
However, batteries tend to decay over time, leaving less and less charge. Eventually, you'll need to be plugged in all the time; unless your charger can supply enough power to keep the phone running, your phone is dead.
Physical damage
Your phone is going to take a beating. Life in medieval times is not easy. It will not only get bounced around a lot more, it will also have to survive humid or wet conditions, and dirt and muck getting on it and in it. You'll want to get a hardened, waterproof pouch for your device.
Superstition
Find some powerful allies, because without them, you're going to lose your phone, and maybe your life. The ability to create light from the palm of your hand, steal images of people, and display YouTube comments will have your phone labeled as a device of the devil faster than you can say "4chan". You may lose your freedom either way, but it's better to be a servant of a strong master than dead, right?
What now?
So now you have a phone, a charger or three, a benefactor, and a fully charged phone with a 3G or better connection to the internet. What now?
Everything
The world is at your fingertips. The internet is full of schematics, DIY, and how-to manuals. Anything you could dream of is available. What's more, you have access to tools like message boards, forums, even VOIP! Where to start? What would give you the most output?
Start smart
Sure, you can't order from Amazon, but you could call historians, engineers, and mining companies. And with the ability to contact people in the future... make them do your work for you!
Begin an investment account. Give a big box to a trusted figure, to be opened at a certain time; do your research online and find the most probable methods of beginning a bank account, accessible to you in the future. Starting with a few dozen copper pieces in each account, instructions to buy or sell, and so on, you can amass considerable wealth.
If your efforts to make yourself rich (albeit through time and the Internet) fail because you aren't able to affect the timeline on the Internet, don't lose hope! There are lots of ways to make money; simply acting as a link between times and/or timelines will surely net you at least a little money. You may be able to make money from selling pictures, or raise funds on sites like GoFundMe or Kickstarter. If nothing else, posting on Worldbuilding will get quite a lot of attention (hey, wait a minute...).
If you're rich, you can pay people to research for you! Even with a limited battery life, you can stick to receiving emails detailing the best steps for you to take, saving battery life since you only need to turn on your phone once a day or so. From there, it's up to you where to point your resources, though comfort, weapons, maps to mining resources, and so on will make your host rich, which keeps you safe.