Traditional brokers
There are tons of players in this market, especially in USA. You have traditional brokers, brokers tied to your bank and a bunch of startups. The easiest is probably a broker tied to your bank, because you probably don't have to wait to fund your brokerage account and can start trading immediately.
Often the older/traditional brokers don't have very intuitive interfaces, it's the startups who do a better job at this. But honestly it doesn't really matter, because you can use reporting services that are different from the services you use to execute your trades. Meaning that you only use the interface of your broker to execute trades (buy or sell), and use third party services to monitor your holdings.
Monitoring services:
Google Finance, Yahoo Finance, Sigfig, Morningstar,... are services allowing you to monitor your holdings. But you can't execute trades with them.
Start-ups:
Then there are a bunch of startups that offer investment services besides the traditional brokers.
Start-up > Robinhood
The most ambitious one is Robinhood, which offers the same service as a traditional broker, but completely free (most of the traditional brokers charge a flat fee and/or percentage when buying/selling hodlings) and with an intuitive interface. They're mobile first, but announced they will be launching their service on the web soon.
Start-up > Acorns
Another popular, mobile-first start-up is Acorns. They offer a lazy-investing service which rounds your everyday purchases and uses the change to invest. It's great when investing is not on your mind, but you still want to invest without realizing it.
Start-ups > Robo-advisors
Robo-advisors auto-invest your money across a bunch of funds picked based on your risk profile. Because the robo-advisers are fairly new, they often have the most intuitive interfaces. These robo-advisors often don't allow you to pick individual holdings, so these services are best when you want to passively invest. Meaning you don't want to look at it very often, and let them do the investing for you.
There are tons of robo-advisor start-ups: Betterment, Wealthfront, Personal Capital, Sigfig, FutureAdvisor,...
Also bigger parties jumped on this trend with their offerings: Schwab Intelligent Portfolios, Ally Managed Portfolio, Vanguard Personal Advisor, etc.
Summary:
It's fun to pick individual stocks, but if you start out it can be overwhelming. Robinhood is probably the best start, they have reduced functionality, but gets you going with an attractive interface.
But soon you'll realize it's extremely hard to beat the market. Meaning that hand-picking stocks statistically gives you a worse return than just buying into the general stock market (like S&P500). So you can decide to just buy one fund with a traditional broker that covers the general stock market. Or you can decide to try out one of the many robo-advisors. They haven't been around that long, so it's hard to tell how effective these are and whether they beat the market.
If you're young, and you believe in start-ups (who often try to challenge the traditional players), try out one of the robo-advisors. If you want to play a bit and are addicted to your smartphone, try out Robinhood. If you are addicted to your phone, but don't want to check up on your investments all the time, go for Acorns. Of course you can combine all these.
Lastly, there are tons of cryptocurrencies which might give you a large return. Tons of startups offer intuitive interfaces to trade cryptocurrencies like Coinbase, Gemini, Kraken. But beware, there is a lot of risk involved in trading cryptocurrencies, it's completely unregulated etc. But definitely check them out.
Oh, and you can also invest by giving out loans through LendingClub, Prosper etc.
Who can you trust?
Above gives you an overview of your options intermingled with some reasoning. But regarding your question "who can I trust" in terms of advice, it's up to yourself. Most traditional broker services don't give you any advice at all, you're on your own. Robo-advisors don't give you advice either, but let their proprietary algorithm do the job. Are these reliable? Nobody can tell, they haven't been around long enough, and they need to go through a bear market (a crash) to see how they respond during rough times.
Some robo-advisors offer you personal consultancy (I believe Sigfig and PersonalCapital) does this (limited to a few hours per year). But obviously they'll try to promote their robo-advisor services.
