SEM is expensive, SEO is less expensive however it takes more time to see results. If I was you I would take a look at the most important factors for a good ranking on the search engines, and I would focus my time and effort on that.
Be really careful with using shady SEO tactics, such as paying for a lot of cheap links. These tactics will impact your site negatively as the google algorithm is very smart about these things.
Spreading your content related to your niche will be helpful to get ranking. Try to create content related to mobile apps, mobile games, features & benefit of iOS and android etc. Make sure that your content is in different format like text, images, videos, infographics etc. Publish your content through blogs, guest posts, social media, video submission websites, inforgraphics submission website etc. By doing this, you will get quality backlink from your niche. It can be really helpful to get ranking on search engines. Once you start getting ranking, you will notice upmark in your website traffic.