Shutterstock is the first stock photo website I got into, in 2009, and their critique forums were very helpful in teaching me about stock photography in a supportive environment. They currently result in about half of my stock photography income.
Read their guidelines carefully and follow them. Photos must be at least 4.0 Megapixels in size. They are known for being particularly picky about things being in focus. Also check out their 50 most popular photos this week.
When you are ready to submit your photos, I would appreciate you using this link. It doesn’t cost you anything and I get a referral bonus and the pleasure of watching your progress! Thanks.