If your garage is attached to your home, you can use your whole house heating system to heat it. But the process is a bit more difficult for a detached garage. A detached garage will not get any residual heat from your home and it will not have connected vents that you can pump air through to heat up the space. However, if you're working in the garage during the colder months of the year, you will likely need some sort of heat source, and the more economical the source, the better.
Set up a portable electric heater to heat your garage if you only need to heat it while completing relatively short tasks. These heaters will heat a small space within the garage and tend to be economical because you will only be using them when you're in the garage. For portable heaters, electric is the safest choice, but be sure it has an automatic shut-off in case you forget and leave it on.
Install a forced air heater if you are going to use your garage for longer periods of time, such as if you use it for a workshop or home gym. In this case, a forced air heating system is more economical than a portable electric heater because you may need to heat your garage for several hours at a time. This type of heating system is typically fuelled by gas.
Insulate your garage if you only want to increase the heat level a little or if you want to supplement the heating system you chose. Adding insulation to the walls will not only keep your garage warmer in the winter, it will also keep it cooler during the hot summer months and help make your garage more soundproof. The most economical insulation choice will be the one with the highest R-value you can find. The R-value measures how effective the insulation is at retaining heat.
Keep the door of your detached garage closed whenever possible to help trap in the heat. This will help make the system you choose more economical. When people enter or exit the garage, make sure they do it as quickly as possible.