Marketing can eat up a large amount of the budget for a business, but if done correctly, should eventually start to pay for itself. You should get back more than you invest in marketing to make it worthwhile, although some results may take some time to come through. Online marketing is vast and there are many areas that you can invest in. Social media marketing, search engine optimisation and pay per click are all forms of marketing that require time and possibly a budget to implement.
You can choose to use internal staff, third party individual or marketing agency. All forms of marketing have advantages and disadvantages and who you choose to use is often dependant on the budget as a marketing agency will offer you great service but it will usually cost more than a using a freelancer.
If you chose to keep the marketing of your business in-house, then you should look to employ someone who has experience and knowledge in this area. The role may not need to be full time to start with but as your business grows you can look to grow your marketing into a department should you need to.