First, decide what tasks you will perform as a housecleaner. If you only want to do "regular" cleaning: vaccuuming, dusting, dishes, floor cleaning, bathrooms--then that's how you have to market your services. Don't offer to do heavier cleaning tasks such as organizing basements or closets, climbing ladders to get to high places, or windows and heavy messes. Make up a flyer offering your services and post them or distribute them to area households--or put a classified ad in the paper offering your services as a housecleaner. Once you get one or two clients, word of mouth will get you more business, if you're good at your job.
Start by finding out what cleaners charge in your area--you do that by calling around and asking what they do and how much they charge for it. You can either decide to charge the average going rate in your area, or you can charge a bit less to stimulate your business. Most cleaners get between $15-25 per hour for cleaning in an average-income area. You'll need to prove yourself at this type of business, which can take a while to do.
Keep very good records--time you arrive for the job, time you leave the job, time spent and tasks accomplished--and make sure you keep all your receipts for supplies and transportation for your taxes later. Give each customer an itemized bill or invoice stating what you've done and how much it costs--and be fair and thorough. If you do this right, you'll have a decent little source of income.
The only things that would concern me is that you are working a full-time job--that will mean your hours for cleaning will be limited. Lots of people will not want a cleaner to come after the dinner hour when they want to relax in their homes; so you may want to consider only being available on weekends for now. You should also investigate getting a "surety bond" (bonding) so you can assure your customers that there is coverage in case of accident or liability. This can be gotten through almost any insurance company. And you should also decide how to accept payment--cash only, checks and cash, or delayed payments--or a monthly fee for regular visits--or whether you want to charge by the hour or a flat fee for certain tasks.
Good luck, and I wish you well.