You should ask them to show you their portfolio. In this way, you will be able to evaluate the result of the previous clients. The SEO company which you choose should offer a reasonable price. It should not be too expensive. It is best that you sign up for SEO company that pay on result. In this way, you won't spend money before seeing a result. However, SEO company that accepts monthly payment may be cheaper and more economical. Normally, pay on result SEO company costs a lot of money. Some SEO company require you to make upfront payment. If you don't know which SEO company to sign up, you can ask your friend to give you recommendation.