Google Adwords can be very competitive, but is definitely the right place to start with selling products online. Start with Search Network and try a bit of Display Network as well. Display network can be profitable, but search is usually more reliable.
Email marketing is also excellent, but you'll need to build your own lists. Make sure you're collecting email addresses: "Sign up and hear about our specials first!"
Other common online advertising disciplines include Search Engine Optimisation, Social Media Marketing, Content Marketing, Affiliate marketing, and Remarketing. Focus on adwords and email for now as they're the easiest to get started with and I wouldn't recommend trying to skill up in everything at once.