There are different classes of motherboards. Which one you need will be dependent on your intentions with the system.
Choosing a CPU will determine how fast your system can open things up. If you are only using simple programs like an Internet browser you would not pick a high end processor. On the other hand, wanting to open up business related software might require something faster.
An HDD determines how much data can be stored on your system. The more files you have, the more storage space you will need. Factoring in the amount of pictures, movies, music, and documents will help decide on which way to go.
SSD's have no moving components. When choosing your data needs and speeds, there is a choice between cost and performance. Choose wisely
The amount of memory your computer has tells you how many things you can open up at the same time. The less memory, the less things you can do. There are requirements for each program.