This process does take some learning, and it's something you want to research thoroughly if you've never done it. Begin by acquiring all needed supplies - even for the smallest of cracks, supply lists can be extensive, but many of these tools will come in handy for other DIY projects as well. A few items you need to gather include but are not limited to:
Hammer
Dust mask
Air compressor
Patching compound
Putty knife
Cement mix
Caulking gun
The repair of small cracks makes for a great DIY project. This generally involves less serious issues, like hairline cracks in mortar joints. You can use some of these materials to repair drywall cracks as well.