Google has approx 80% of the deskktop browser market. Firefox is a better browser and protects your privacy. But let’s concentrate on what most people use. Google uses your mobile site to index your site and rank it. It’s important to get that right. This is certainly the case when it first indexes your site, what happens after that is less clear but Google always want an excellent mobile site. Many older sites are understandably not optimal in this respect.
A useful overview here.

The Yoast SEO plugin has a free version and is good. Get in touch if you want more information on that.

Adding content and posting regularly is going to help your SEO. Should I blog for SEO is a question I get asked. If you have competitors (such as businesses) people do search with phrases like ‘Accountants in Cardiff’ – then it’s probably a very good idea. If you are a charity blog as much as you like but not really for SEO. People don’t tend to search for ‘Charities in Cardiff’. Possibly, but unlikely. They will know you and enter the charity name and find your website that way.