Most bike maintenance is quite straight forward if you have to tools and know what you are doing.
If the bike has not been used much and has been in a relatively dry environment in the garage, then it shouldn't need much to get it going again.
Check the tyres for cracks in the sidewall and/or the rubber compound being hard/brittle - they will need replacing if this is the case.
Check the chain for rust, but assuming it isn't too bad, should be easy to clean up and re-lube following a guide online.
Check the brakes work (and check the brake cables for rust).
Check the gears change (and check the cables for rust).
If you find yourself in the position where the chain/cables/brakes need replacing, its probably best to have a shop do it until you are more familiar with maintenance. Otherwise, you should be able to do it all yourself.