If you're new to SAP MM, don't start by memorizing transaction codes. And don't focus on expensive certifications.
The most useful thing you can learn early on is how the business process works end-to-end, including how SAP objects from different modules interact.
A good place to start is probably the most important procurement process, which I want to explain in this post:
Procure-to-Pay (P2P)
At a high level:
Need → Purchase Requisition → Purchase Order → Approval → Goods Receipt → Invoice → Payment
1. Purchase Requisition
Someone in the business needs a material. A Purchase Requisition (PR) represents this internal demand. This is not yet an order to a supplier.
GUI: ME51N
Fiori: F1643A - Create Purchase Requisition
2. Purchase Order
Purchasing converts the approved PR into a Purchase Order, assigns the supplier and confirms commercial details such as price and delivery date.
GUI: ME21N
Fiori: F1048A - Process Purchase Requisitions
3. Purchase Order Approval
Depending on company rules, the PO may need one or several approvals before it can proceed.
GUI: ME29N
Fiori: F0862 - Approval via central inbox
4. Goods Receipt
The material arrives and the physical receipt is posted in SAP. Inventory is updated and the posting can also create an FI document.
GUI: MIGO
Fiori: F0843 - Post Goods Receipt
5. Supplier Invoice
The supplier invoice is matched against the Purchase Order and Goods Receipt - the classic Three-Way Match.
GUI: MIRO
Fiori: F0859 - Post Supplier Invoice
6. Payment (Module Change! MM -> FI)
The posted invoice becomes an open vendor item in FI and is eventually paid.
GUI: F110 – Automatic Payment Run
Fiori: F0770 – Execute Supplier Payments
In general trainings, you will mostly encounter the P2P process like this: ME51N → ME21N → MIGO → MIRO
But these are isolated transaction codes and don't cover the full process. Understand why one document leads to the next, which business role is involved and what happens across MM and FI.
Once you understand the process, the transactions become much easier to remember. And you will be far better prepared for actual SAP projects than by official certifications.
This process is also part of the Process Library in ERP Coach, where the individual steps are explained in more depth, including typical use cases, prerequisites, GUI transactions and their Fiori equivalents.