Need help in Professional Military Staff

Am new to this game. I need soldiers and officers to create an defense unit but I don't have enough staffs. How do I recruit ?

When you purchase a piece of military equipment, the required crews are taken from your available labor pool.

Soldiers are automatically recruited from Low Level Workers.
Officers are automatically recruited from Medium Managers.

Be careful with buying to much military too quickly, as if there are not any spare workers or managers, they will be taken from your corporations. This can cause disruptions to your profitability, and in extreme cases bankruptcy.

Can we assume you are setting up "mobile" units? If so you need professional soldiers and officers which can only be purchased through "direct trading" tab on the left panel using gold coins.

In general you can operate a military quite well with the other units available which purely recruit their officers and soldiers from your general workforce. The only units which require professionals are the "mobile units"