The most common problem: The print won’t stick to the bed or the first layer is uneven.
The Fix: Make sure your bed is level and clean. Consult manuf. Instructions or user videos on leveling the bed. Wipe the build surface with rubbing alcohol. If you are using a glass bed, sometimes you might want to rub it with glue stick to get the print to adhere. Cheap hairspray also works well; it is just enough glue to hold down a print.
The second-most-common problem: tangled filament.
The Fix: make sure the filament is properly wound on the spool and not loose or tangled at the start of every print.
The third-most-common problem: clogged hot end.
The Fix: Many printers come with a tiny, thin needle (usually an acupuncture needle). With the hot end heated up, run this up into the nozzle from the bottom. Push it in and out a few times and clean it off. This often fixes the clog. If unsuccessful, heat up the hot end to 250 deg; when it reaches temperature, set it to “cooldown”. As the hot end temp reaches 180-190, retract the filament (forcefully if necessary). Snip off the last inch or two and put it back in. Try printing again. This is called a “cold pull” and will usually clear whatever gunk or dust has accumulated in the nozzle. If all else fails, remove the nozzle (read manuf instructions) and heat it up until all traces of plastic have melted out.