In many cases if you have never located your heating systems filter it can be a challenge to find it. All systems are different so we will do our very best to point you in the right direction. Typically your filter is going to be somewhere near the blower (fan), this is because one of the main purposes of a filter is to protect the blower (fan) from damage. This applies to all different types of furnaces, including gas furnaces, oil furnaces, electric furnaces, as well as air conditioners.
Step 1 - Make sure the fan is not running, you can do this by setting your thermostat to the off position and turn off power completely to the unit.
Step 2 - Locate the service panel (main cover), usually on the furnace's front or side. The front of most systems will have a label that gives details on removing the cover/panel. In most cases you will not need tools to remove it...you will just have to pull up on the handle and it will pop out.
Step 3 - Now that you have removed the panel locate the filter. You should be able to see the frame of the filter sitting horizontally or vertically near the blower (fan). If you find the filter slide it out and take note of the size, replace with the filter of your choice from http://www.rememberthefilter.com/
If you couldn't locate your filter by following the steps listed above work you way backwards from the return duct this is the one that brings air into the system, remember since the filters main purpose is to protect your system and keep it running efficiently it is going to be somewhere before the blower (fan). In some situations there may be no furnace filter present. It may have been removed instead of replaced, or it may have never been installed in the first place. In this case replacing the filter will be a greater task. If you can locate what looks to be where the filter would go you will need to take measurements to be sure you order the correct size. If you can't locate the location of the filter we suggest you consult a local reputable HVAC company to do an inspection of your system and inform you of the size of filter you need and where it goes.
Note: You should never run your system without having a clean filter in place, this can save you money in the long run...not to mention keep you and your family breathing healthier, cleaner indoor air.