For the best Web Promotion you initially have to get a clear concept of SEO, SMO and SEM. First of all SEO and SMO are parts of SEM. SEM consist of both organic and paid search engine optimization methods. In my opinion you should go with the organic search engine optimization first, then analyse the result and the user interactions, which can give a complete knowledge of that on which keyword you can invest money to get the higher ROI.
So start with SEO and SMO, as they are free and SEO leads to greater link building, where as SMO leads to social signals, and social brand awareness, and you get the best user interaction on your webpages.