Give each language its own URL
Serving both languages on one URL based on cookies or browser settings stops Google indexing one of them. Put Arabic and English on separate paths, such as the root and /en/.
Link the two editions with hreflang
hreflang tells Google the two pages are the same content in two languages, so each user sees the right one. Each page must reference the other and itself, or the tags are ignored.
Don't auto-redirect by browser language
Automatic redirects can hide one edition from Google's crawlers and annoy visitors who want the other language. A clear switch button is better.
Write Arabic, don't translate it literally
Literal translation produces headlines no one searches for. Find the words your Arabic audience actually uses, which often differ from English equivalents.
A title and description for every page in every language
An English page with an Arabic title, or the reverse, is a common template-copying error. Review titles and descriptions by hand after each release.