NEW YORK ( TheStreet) -- One thing that Jim Cramer always harps on to RealMoney readers and viewers of his "Mad Money" show is the importance of doing extensive homework before investing in any stock. Nowhere has this been more important than in the energy sector in 2013. With the Dow reaching all-time highs and with energy being one of the strongest sectors in that rally, you'd think that any investment in energy would be a winner, but that's very far from the truth. If you've picked right inside the energy sector, you've seen tremendous upside already in the new year. If you've picked wrong, however, you're seeing stocks slide to some of the worst prices they've had in years. Have a look at the video above that I shot with Jim. It differentiates the good from the bad and stress the necessity of doing some very intense homework before picking stocks and buying them. Follow @dan_dickerThis article is commentary by an independent contributor, separate from TheStreet's regular news coverage.