Welcome to AVWW
while in the settlement, there are two ways you can construct a building:
Go to the green ilari stone and press [E]
or alternatively press [V], go to Continent Status, then go to 'Guardian Power Scrolls', and finally click there.
If you are short on a material, or dont have any npc's in your settlement with the respective career at the respective level, you can right click on a building scroll to see what you are short.
To obtain Guardian Power Scrolls, you have to do missions (both worldmap visible missions, as secret missions).
All materials are automatically put in the settlement stockpile [C] and as such dont take up space in your inventory.
There is no limit on the amount of materials that i know of (there probably is one, but you'll either never hit it or by the time you do it wouldn't matter since you'd have such a huge surplus that you wouldnt know what to do with it anyhow).
You can't access buildings in the settlement at this moment, but thats alright since there isnt much use in doing so at the moment anyhow.
Spellgems can be remade at the spellcraft station at the level you have learned them at free of charge, learning them for the first time / upgrading them however does cost materials.
So you can drop those if you want to make room (not that you'd need it, i think you have around 100 slots of free room in your inventory, i dont even think there are that many usables to start with).
Enchants have their own inventory, 100 slots for storage, and 5 for wearing.
This storage can easily overflow, so ditch the ones you have better versions of / dont need.
To get rid of items in either inventory, just drag them out of the storage and drop them on the ground.
Did this answer your questions?
edit:
ninja'd by toll, blegh
wow, there is 17 minutes between my post and Toll's, i'm slow!