needles should be changed between each project or after 8 hours of sewing time--which ever comes first- and sooner if you start having uneven or skipped stitches- some fabrics will dull the needle faster than others.
you should clean your machine every second bobbin---and if it's a new-fancy machine---follow the maintenance guidelines in your manual--they tell you how to clean it and oil-if it needs to be oiled-
if you start having problems with stitch quality or other issues it may have to go to a shop-but you should be able to do 98% of the maintenance yourself for years. i do have a computerized machine- which i've been taking care of for a long time- it's been in to a shop once in the past 5 years--had a problem with my reverse ... but that was a whole different matter :)