Most programs have trainers who handle different types of riders. The head or top trainer will take the more advanced riders or riders who are ready to show. Then there may be a trainer who handles the “lesson program” type riders - the once-a-week-ers or the beginners. There may be an in-between trainer, too, depending on how big the program is or how specialized the lesson program trainer is. There’s generally an agreed upon handoff point, where the student transitions to the next trainer, although this is usually based on feel rather than a hard and fast rule.
Consistency is key with new riders, so I would advise having an assistant who does all beginner lessons or lesson-program lessons, transitioning riders to you as they are ready to show or as they are more advanced in skills and are ready for your instruction.
This situation works best with an assistant who is not looking to become the head trainer…not yet, anyway. Look for a young pro who needs to develop their teaching skills and will benefit from running a lesson program for a while. Potentially, if the program grows enough, that person could step into a “second in command” sort of role as they grow and develop, bringing on another beginner trainer once that happens. But you don’t want to hire someone that will be resentful of the “good” riders being passed on to you. You need someone who understands what the position is and whether or not there’s a possibility of that growing in the future (which is entirely up to you and the size of the program you want to have).
As far as pay, it’s done all sorts of ways, as others have mentioned. A % of lesson fees is common, although the % varies, as is a set hourly wage, or even a weekly salary that includes an expected workload (which may include exercising or tuning up lesson horses or giving a hand with cleaning lesson tack, etc.).
Make sure that your pay is commiserate with the type of person you’re looking for. Are you looking for someone to just handle beginner lessons? Maybe a set $15/hour is applicable. Are you looking for someone to truly grow into being your assistant trainer? Perhaps a % commission with a plan for % increases as their skills and program increases is more enticing to the type of person who would fit that job.
Ultimately, you need to first decide what you want your program to look like and what types of trainer(s) you want under you. There’s no right or wrong answer to that question, but you WILL save yourself a lot of turnover, pain, and headache if you answer those structural questions first.