If you have to narrow it down to three points I would suggest the following ....
1. All good marketing starts with the customer. You must have a solid understanding of the needs of your customer and how you can deliver value to them. To get to this point you need to engage in marketing research and segmentation efforts. While these are basic tenants of marketing they are tough to do well.
2. Once you have an understanding of the customer you must have an understanding of the market. The vast majority of companies never do manage to put together a market map. A market map details the relationships and values found within your market. The market map plays an important role in helping to identify market segments, positioning, and competitive presence. All of these elements need to be understood in addition to the customer knowledge noted in point one. Completion of the first two steps helps to build a situational analysis. Basically you cannot go forward until you understand where you are.
3. With an understanding of the customer and the market in place as a startup the final piece of the puzzle is your people. Understanding where you are and where you want to go is terrific but you need the right people to get there. Hiring the right people to lead your marketing efforts is a challenge that must be met in order to be successful.