Alright so getting through that sealed gate in the royal capital can be a bit tricky, but here's the main path you'll want to take. When you reach avenue balcony grace, look for this massive dragon corpse draped over the buildings - you really can't miss it. The key thing most people don't realize at first is that you actually need to climb up the dragon's wing to progress, which gets you to some higher platforms and eventually the erdtree sanctuary area.

From there you'll need to keep working your way up through the architecture, and eventually you'll reach the elden throne. Just a heads up, make sure you've grabbed two great runes and done what you need to at roundtable hold before trying this, or you won't even be able to enter the capital in the first place. The main challenge here is going to be morgott the omen king - he's the big story boss you'll need to defeat to move forward. Once you've taken him down, you'll get access to the lift behind his arena that takes you up to the mountaintops.

Tip for you - don't waste time trying to open that gate directly near the erdtree avatar. The intended path is actually going up and around using that dragon wing i mentioned. Focus on vertical exploration and you'll find your way through just fine.