Start at the bottom. Sorry to break that to you, but if you’re looking for a get-rich-quick guide to the publishing industry, you’re not going to find it here. You should first apprentice to a current acquisitions editor to get to know the ins and outs of acquisition editing. Duties may include copy editing, as well as the time-honored tradition of getting coffee.
Work for a literary agent. You don’t necessarily have to work in a publishing house to learn about acquisitions. These days, the literary agent is a very important part of the process--a kind of first line of defense that will weed out the bad books from the better ones. This is similar to the process of acquiring books for a publishing house. There may even be editing involved, as agents try and get books into the best shape possible before shopping them.
Work freelance. Another option is to become a freelance editor--helping authors to get their work in shape before shopping it to agents or publishers. Technically, this isn’t acquisition editing because you’re not working for a publisher, but it will provide needed experience if you want to work for a publishing house.
On that front, you could start up a small publishing house of your own. It is cheaper these days to start a publishing business via print on demand. Rather than absorbing the cost of printing up 1,000 books in bulk that may never sell, each book is printed as it’s ordered. Often print-on-demand publishing houses are started by writers publishing their own books who then devise an imprint for the book. You can then become a full-fledged acquisitions editor. However, you’re going to need a good book designer and marketing plan to attract quality writers.