Mekanism: Getting Started with Machines and Materials
Mekanism is a large tech mod for processing resources and generating power, and it begins with a few core materials. This guide covers the foundation you build before the advanced machinery.
Core materials
Mekanism adds its own metals — Osmium, Tin, 
Lead, Uranium and Steel — smelted or blasted from their dusts and ores. 
Infused Alloy is the first crafting alloy and appears in almost every early recipe, upgrading later into 
Reinforced Alloy and 
Atomic Alloy.
Control circuits and energy
Two building blocks recur everywhere:
Basic Control Circuit is the base component; combine it with Infused Alloy to make an 
Advanced Control Circuit, then Elite and Ultimate circuits with better alloys.- The
Energy Tablet (redstone, gold and Infused Alloy) stores power and feeds into energy cubes and cells.
Machine tiers
Mekanism machines come in tiers — Basic, Advanced, Elite and Ultimate — and you upgrade them in place with Tier Installers. A 
Basic Tier Installer is made from redstone, Basic Control Circuits, iron and Oak Planks. Start with basic machines and installers, then climb the tiers as you produce better circuits and alloys. From here, see the machine-tiers, energy and gear guides for each system in depth.
What you are actually starting
Mekanism is 368 pages across 200 blocks and 168 items, and its recipe list is one of the largest anywhere: 440 sawing recipes, 322 enriching, 313 crushing, 226 pigment extracting, 204 crafting and more than twenty other categories.
None of that has to be learned up front. The early game is ore processing, and that means three machines: an 
Enrichment Chamber, a 
Crusher and an 
Energized Smelter.
Ore doubling first
Enriching and Crushing between them carry over 600 recipes, and the vast majority are ore and material processing. Building those two machines before anything else roughly doubles every ore you mine, which funds the rest of the mod.
Beware version-specific chapters
Seven recipe categories exist only on 1.21.1: pigment extracting (226 recipes), oxidizing (25), chemical conversion (24), dissolution (23), pigment mixing (16), washing (7) and chemical infusing (4). A 1.20.1 world has none of them.
In the other direction, 
Painting carries 976 recipes on 1.20.1 and 176 on 1.21.1.
What to build after ore processing
Energy production and the transmitter network, then the chemical line if you are on 1.21.1. The tier system for machines applies uniformly, so upgrading is a matter of material rather than of new recipes to learn.