With web development it's easy to feel you need to know about everything.
On the front end it's essential to understand markup semantics, accessibility issues, DOM scripting, CSS and the multitude of cross-browser bugs. Then you need a basic understanding of interaction design, how interfaces work and how to structure a site. Plus, a sprinkling of design savvy is handy too, knowing your line-height from your ligature will keep you in good graces of designers. Then, if you venture to back-end code you might need to use PHP and Apache one day, ASP.NET the next and Django on Friday. If you're feeling optimistic you might want to take in search engine optimisation while you're there.
It can all seem a bit daunting if you're new to all this.
One of the most useful things I learned, fortunately early on, is that sometimes it's best to hold information lightly, but hold your information sources tight. You don't have to know every PHP function when the manual is online 24/7. Hunting a browser bug is easier if you have quirksmode, position is everything, the Sitepoint Reference and Firebug on hand, and with the web you do. Learning how to navigate MSDNs labyrinthine architecture is an artform all its own, of course, but the information is there if you can understand the mindset... Even if you can't there are great conferences, helpful forums and local networks to point you in the right direction.
Knowing everything isn't necessary. Knowing where to find what you need. That's essential.