SEM stands for Search Engine Marketing and covers a range of topics from Searching Engine Optimisation (SEO) to Pay per Click (PPC) and much in-between.
If you want your website to perform well on the search engine results pages (SERPS) then you need to ensure that you have a level of Search Engine Marketing that is correct to the current level of competition and the results you hope to achieve.
You can carry out certain elements of online marketing yourself, even with little knowledge but this can be risky and can actually end up being detrimental to the rankings of your site.
I would strongly recommend that you take advice from a specialist marketing agency who will be able to assist you with the process and advise you on what they think you need to do in order for the website to perform well.
Budget is always the main issue when it comes to marketing, so you need to have an idea of what you wish to spend and be aware of the restrictions you may face if the budget is particularly small.