The first thing to do is decide what part of VOIP marketplace you want to serve. Here are some possibilites:

• Prepaid Cards
• VOIP consulting
• PBX sales and service
• Call Center Solutions
• Independent Sales/Service Agent for existing VOIP service providers
• VOIP Provider services
• etc.

Within each category there are many choices. For example, for Prepaid Cards:

• Retail prepaid cards from existing wholesale providers
• Start your own brand of prepaid cards using services from existing wholesale providers
• Start a new prepaid card provider company
• Create new software package for prepaid card services
• etc.

Some general suggestions:

• Pick an area that plays to your strengths. For example, if your strength is sales and marketing, pick an area where you can leverage those abilities
• Learn all you can about the maketplace
• Attend industry tradeshows
• Read industry magazines, blogs, forums, etc
• Read books
• Do market research - talk to your potential customers
• Ask questions
  • Tradeshow exhibits usually have knowledgable people available ready to answer questions
  • Many websites (like this one) let you ask questions
• Test the waters — to the extent possible try before you buy, test the waters before making large commitments of time or money