• GLJ Research analyst Gordon Johnson downgraded SunPower to a Sell and slashed the price target from $1.61 to $0 last Thursday.
• Citigroup maintained its Sell rating and lowered the price target from $2.50 to $0.25.
• Mizuho downgraded SunPower from Neutral to Underperform and lowered the price target from $4 to $0.50.
• JPMorgan Chase analysts led by Mark Strouse stated in a research note that they see the halt as a permanent stoppage for SunPower. 

SunPower shares fell in early trading Monday, but have rebounded higher on heavy trading volume. According to data from Benzinga Pro, more than 40 million shares have already been traded in Monday's session and 43.9% of shares are being sold short. Stocks with outsized short-interest can experienced increased volatility, especially when combined with heavy trading volume.

Buying shares is typically done through a brokerage account. You can find a list of possible trading platforms here. Many will allow you to buy ‘fractional shares,' which allows you to own portions of stock without buying an entire share. For example, some stock, like Berkshire Hathaway, can cost thousands of dollars to own just one share. However, if you only want to invest a fraction of that, brokerages will allow you to do so.

If you're looking to bet against a company, the process is more complex. You'll need access to an options trading platform, or a broker who will allow you to ‘go short' a share of stock by lending you the shares to sell. The process of shorting a stock can be found at this resource. Otherwise, if your broker allows you to trade options, you can either buy a put option, or sell a call option at a strike price above where shares are currently trading – either way it allows you to profit off of the share price decline.