What to prioritise when looking at SEO?

I am just starting out and slowly learning SEO. I want to create a test site to see if I can experiment and learn as I go.

From what I can tell, first place to look at is on site SEO. Titles, h1's etc - are meta tags still important these days?

After this is where I get a little lost. Backlinks are important, do I focus on social (web 2.0?) links, youtube or building up a PBN with some authority domains?

You can do both for backlinks. Initially start with web2.0, in between gradually build your own pbn.

Start with web 2.0, add some social signals, bookmarks, authority profiles, after a few months you can buy or build your own pbn. And keep adding quality content on your site. Titles, h1 are important part of on-page seo, but meta tags - not such important by my experience. Actually you can find a lot of great backlink guides for beginners here on BHW.

Anything thats producing results bud...

Before you do anything else, find the right keywords. If you do your keyword research right you should be able to rank with good on site SEO and only minimal investment/work on backlinks.