For convenience, I will try to share my knowledge step wise.
1. Get yourself prepared. Seek help from friends and family. Help could be in the form money, ideas, inspiration, and motivation.
2. Make a general business plan. You should be well aware of upcoming real life complexities. Complexities can refer to marketing difficulties, SEO difficulties, monetary difficulties and more. Making a list of actions can be very helpful. Where to start, what to promote, how to generate revenue are the questions you should be able to answer yourself before jumping into the field. Get ideas from everybody you know. Out of 100 suggestions, odds are 10 would be very helpful tips. Reading blogs, reading news on already established classifieds sites can be very helpful.
3. Buy a domain name. Always use relevant domain name. Domain name should reflect what your website contents will be in future.
4. Buy hosting service. Hosting service is what makes your website visible to the worldwide internet users. GoDaddy.com is really good place to buy hosting services. I found using Linux hosting is better.
5. Buy a software package from a well established script seller. There are quiet a few in the market. Find the one that offer your needs. Simply use any search engine and type ‘classifieds script’.
6. Most companies offer free installation and if they take certain fees, request for a free installation.
7. Now, you can browse your website. Software company will provide you the access to administrative section for your website (c-panel). You can edit your website like the way you want very easily from c-panel. Most software companies will provide you with user manual to edit your websites.
8. Edit the website exactly the way you wanted and get ready for further steps, marketing and generating revenues. Refer different sources and article for this purpose.