The best marketing strategy is to make things people want. Find a gap in the market which you can fill. Solve a specific problem for a specific person. Build a product or service which meets this need. Create a minimum viable product (MVP). The MVP is a very cheap prototype of your product which meets the customer’s requirements. Then you test the product with real customers and get feedback. Use the feedback to improve the product offering until you have something decent.
Next you take the product to the specific market you designed the product for. Create awareness through word of mouth and any other affordable process. Focus on getting your first ten customers. Gather and act on the feedback from your early adopters.
Create a network effect by encouraging your initial customers to tell their friends. If your product genuinely meets a need for this market you will have no problem with marketing and sales. Just make things people want.