There are a number of readily available web design tools that you can use nowadays for free. Popular ones like Canva & Vectr are good options because they're free and are user-friendly. Although, if you have the means, investing in more premier options like Photoshop and the Adobe Dreamweaver is a definite must.
One project that I'm working on right now is a website whose design needs to look highly professional because it'll be a domain for lawyers, paralegals, and the like. This is why I opted for darker hues and less creative fonts; I made it so that everything was readable and easy to understand.
Although this was not the main point of the thread, I think it's unavoidable to talk about web design without taking into account the functionality of everything within the site that's directly under the premise of web development. More modern sites are not just attractive; they need to be efficient too!
With my current project, one thing I needed to get right was the implementation and automation of
electronic signatures because it is the primary aspect of the aimed service as a whole. Making contracts more seamless and professional. This, I think, is the main thing that you should take into consideration with your design, you also need to remind yourselves about the main goals and services you're trying to deliver so that you can come up with a compelling design that serves its purpose.
Hope this helps, good luck!