There are several ways to approach this problem. Broadly speaking they fall into 3 categories:

1) Get more qualified leads (SEO, Google My Business, Google Ads, Social Media, Offline Ads etc)
2) Increase onsite conversions (Improve conversion via better usability, messaging, copy and general design)
3) Improve follow up (email marketing, retargeting or social media)

I would suggest making sure your website is up to spec first. Many businesses will find it easier to double sales by improving onsite conversions than doubling there leads. By doing this you improve the ROI on any ad spend or time invested in generating leads too.

Getting more leads is great and should be done once your conversions are up, but is typically an ongoing expense. There is no ‘one size fits all’ answer here, but generally service industries should focus on Google My Business and Google Ads, products can do well with Google Ads, Google Shopping, SEO and in some niches (such as fashion and art) social media can be very effective.

One of the best ROIs for most businesses is email marketing. Once you have the lead following up makes all the difference. Email can also help generate a lot of repeat business.