I have been unable to mine some blocks, (ie: obsidian, molten rock) but I'm not very advanced in the game yet. What are the simplest pickaxes to get that can mine strong blocks relatively fast?
The pickaxe you're going to want to go for is the Reaver Shark. It's a pickaxe that can mine all the easy mode ores, and the beginning hard mode ores, and can be acquired from the very beginning of the game. It can be obtained by fishing in the ocean, with a 1:50 chance of getting it with 100 fishing power.
The Deathbringer pickaxe is the fastest pre-hardmode pick (besides the Bone Pickaxe, although the Deathbringer is much easier to obtain and can mine more blocks) for most common blocks, with the Nightmare pickaxe being only slightly slower.
Both of these pick axes require 12 crimtane/demonite bars and 6 tissue samples/shadow scales. Crimtane/demonite ore can be found in very small veins underground, or farmed from easy bosses such as the Eye of Cthulhu.
However, I recommend that you summon the bosses manually, rather than breaking the Crimson hearts/shadow orbs. This way you can set up an arena for the fight that suits you, without the restrictions of the chasms of your Crimson/corruption. I found that a dirt bridge over the top of the Crimson/corruption works well enough, as long as you have the aid of rocket boots or one of their upgrades.
TL;DR: farm Eye of Cthulhu for crimtane/demonite, defeat Eater of Worlds/Brain of Cthulhu, enjoy one of the fastest pre-hardmode pickaxes!