Some brokers have minimum deposit requirements, while others may require a minimum balance to access certain advanced features or trading platforms.

If a broker is offering a new account promotion, there may be a minimum initial deposit requirement to qualify.

If you're trading frequently — more than weekly — you'll want an advanced broker that has powerful platforms, innovative tools, high-quality research and low commissions.

Those who trade monthly or yearly will want a well-rounded broker with a user-friendly interface, helpful customer support and competitive pricing.

Robo-advisor services use algorithms to build and manage investor portfolios. For a very low fee, they'll create a portfolio of ETFs based on your investing goals and risk tolerance, then rebalance it as needed. Many also offer tax-loss harvesting for taxable accounts.

If that sounds too hands-off for you and you want to manage your own investments, choose a self-directed account at an online broker.

Low Cost: Mutual fund/ETF investors want access to funds without commissions or fees; stock or options traders want low commissions and no added fees for inactivity, tools or research.

Platform: If you plan to trade frequently, you likely know what kind of tools you'll use most and what you want out of a platform.

User interface: Tools should be intuitive and easy to navigate.

Premium research: Investing, particularly frequent trading, requires analysis. The broker should provide extensive information to help you select the investments for your portfolio.