Select keywords for your website - how?

Start with your pages, not a tool. Write down what each page is actually for, then think of how a customer would type that into Google. Run those seed phrases through whatever keyword tool you have, free ones are fine at this stage, and look at the suggestions it spits out. You're after terms that match the page and that sites your size actually rank for. Volume matters less than people think, a 200/month term that converts beats a 10k one you'll never crack. Then give each page one main keyword and a few close variations. Two pages chasing the same term just compete with each other.