Employees' Role
While Maxis may improve the AI of Employees in some future patch, for now I can say that they're definitely not meant to take care of everything. Left on their own, there will be very few sales. You will not get Retail Perk Points for their interactions with customers. This means it's best to have your own Sim working customers, while they take care of restocking and cleaning. It's not a bad idea to have them answer customers' questions, either, but when they get tied up talking in group conversations, a lot of sales can be lost if you rely on them to ring up all the time. You should also watch for their roles to shift and manage them every day, sometimes checking on their assigned role multiple times during a shift.

Improving Skills
As previously discussed, employees can go up in level based on what they're doing in your store and gaining experience in those areas. It is in your best interest to keep employees happy for this reason. They'll work harder for you, and you'll waste less time on employees that have low Skill in the areas you need - particularly work ethic. Here are the three Employee Skills and how they can be improved. It's fairly obvious, but important nonetheless.

Work Tasks
You should assign your employees' work tasks based on what needs done. Simply click the employee, then assign them to something.:

  • Clean Store - Do this whenever an uncomfortable moodlet pops up from the store being dirty. Your Sim doesn't seem to be able to clean things, but employees can take care of that.
  • Restock Sold Goods - Do this whenever there are multiple things that need restocking, in order to open up your customers' options.
  • Answer Customers' Questions - When nothing else needs done, this is a good choice. Your employees will work customers a little, and possibly raise their sell bars.
  • Ring Up Customers - Instruct an employee to do this when you see multiple customer sell bars filling at once, or whenever you've lowered prices to move products. They will indeed help, just not that reliably if they get tied up in group conversations. 

Employee Job Levels & Satisfaction
Employees have job levels and set pay that scales a bit based on their Skills once they reach Executive Sales Associate. When an employee gains a Skill level, their Satisfaction will fall if they feel they are not being paid enough. You can keep Satisfaction around neutral or above by using 'Check Up On' and 'Praise'. Don't hesitate to 'Berate for Slacking' as the option will only be available if they have been doing so. That option along with criticize gives some skill experience in work ethic. If their pay is too low and they're very dissatisfied, this might be a bad idea though. They should get a promotion when it's due. Here are the rough pay levels and guidelines for when an employee should be promoted to that level. This is about what it will take to get an employee to 'Satisfied' or 'Very Satisfied' along with regular use of the two positive reinforcement socials at your disposal.