@yying You really don't want anything that is "Robinhood style" - while they have no commissions, they do have a spread that isn't advertised. And their orders take longer to execute so you need to use a limit every time.
Schwab is your best option as an American expat. They have international accounts and since you're in the UK, you fall under the jurisdiction. You won't have any commissions and will have access to all their research and features. As far as a quality discount broker goes, Schwab is the best (and they're the only one that offers brokerage services to Americans outside the US).
Interactive Brokers is also an option but you'll have to pay commissions with them so unless you're a high volume trader, I don't see it being worth it. A lot of people like Degiro, but even though you've never lived in the US, as a US citizen you are still under FATCA which means a lot of banks or brokers won't take you on as a client, Degiro being one of them.
As for training, I would learn some basic macro and micro concepts and focus more on fundamental analysis rather than technical (technical isn't as good in the market we're in right now). Investopedia is a great place to familiarize yourself early on. Schwab also has some training for beginner investors that teaches you how to invest properly (timing, valuation, strategies, etc.), but you'll need to open an account with them for access to all that I believe. You can also start reading financial newspapers/publications. I like The Wall Street Journal but that can be expensive if you're not getting it through an organization (my uni pays for mine). CNBC's articles are mostly free and can give you a bit of an overview on individual companies and sectors. And while you've probably heard of Bloomberg (their subscriptions are also expensive), most people don't know that you can watch Bloomberg TV live for free online and get a great financial overview there. If you have a Firestick or any type of streaming device, you can download their app and watch it. Otherwise, you'll need to go to the livestream online (not sure if you need a VPN to change your location to US or not).
Good luck! And for an early tip, take a look at ERIC (Ericsson) - I think there's some great growth potential there!
