Select a hosting provider.  We use Hostgator in the US, which as a the Baby Plan and Business Plan, both of which provide an unlimited number of sub-domain websites.  So, for this price, you can host an indefinite number of websites.  However, these are on shared servers, which means you could have performance issues.  This is not for high performance and high bandwidth websites, but just to start out, this is perfect.

Link: http://www.hostgator.com/shared-compare

Cost: +/- $10 per month

Time: 5 minutes to buy