The cheapest (free) type of SEO is on page optimisation, there are tons of guides here on how to properly optimise your META titles, descriptions, page structure and on page content. Once your on page SEO is sound, then you need to build high quality and diverse backlinks, social signals, quality PBN links. How many links you will need will depend on your niche and keyword difficulty. Also remember to add fresh, new and relevant content to your blog targeting keywords.