Fiddle
Strings:
For quality of sound and playability replace your strings every 6 mo. to a year. If DIY only replace one string at a time. Avoid loosening all of the strings at the same time which could cause the sound post to fall.
Keeping the fingernails of your left hand completely trimmed could help minimize string wear.
Clean rosin build-up off of your strings with a cloth, paper towel or paper tissue, but avoid the kind containing lotion. Avoid touching strings with fingers where the bow makes contact, this could tranfer oil from skin to strings.
Instrument Body:
Always keep the bridge in its proper upright position and location. Be aware of any possible bridge warp. If warp gets bad enough a new bridge is necessary.
If the wooden tuning pegs are too difficult to turn, or they won’t stay in place they may need to be serviced.
Be aware of the soundpost having fallen, (you can hear it rolling around). If this happens immediately loosen all strings and get your fiddle serviced.
With normal play, rosin dust accumulates on the top of your fiddle under the strings. Over an extended period of time, this dust may permanently stain the finish of your instrument. Regularly clean rosin dust off of your fiddle with a cloth, paper towel or paper tissue, but avoid the kind containing lotion. Do not let oil or polish contact the strings or bow hair.
Occasionally look over your fiddle for any separation of glued seams, especially near the end pin. Also, be aware of any new cracks appearing in the top.
Never leave a fiddle in a hot car. The temperature inside a hot car can be sufficient to soften the glue that holds it together, especially at the base of the neck and the endpin which are under tension from the strings. A small amount of damage can be very expensive.
Bow:
Avoid touching the bow hair with your fingers, this could tranfer oil from your skin which is bad for the hair. Avoid touching fiddle strings where the bow touches them. Also avoid having allowing food or drinks near your bow.
Never over-tighten the bow, too much tension can weaken points where hair is attached, or lead to warping of the stick. Always loosen the bow when stored.
Rehair (or replace) your bow when a significant amount of hair has been lost, or its performance is degraded.
Rosin:
Rosin needs to be applied to the bow hair as needed. A few strokes to a dozen should normally be sufficient. (If the bow hair is brand new it will require much more). If the rosin cake is brand new it will have a glossy smooth surface and the rosin won’t apply properly. Take some sharp pointed metal object and lightly scratch the surface in a criss-cross pattern. This will allow the bow hair to begin wearing into the rosin.
Rosin is a very brittle substance. It will easily shatter if dropped or crushed making a powdery, sticky mess. It will also become sticky in hot humid weather.
Accessories:
You can find many needed accessories online, usually from the same store. I have no affiliation with any stores or product brands, but I’ll share my preferred products and recommend stores that seem to be reasonably priced.
BTW, if you’re an adult you probably have a full size fiddle (violin). Sometimes this is listed as 4/4.
Southwest Strings carries well priced strings, rosin and many other items.
Prim Strings makes a steel core string and is my choice for fiddling. They are bright sounding and hold up well over time. There are 3 gauges; medium, soft and orchestra (heavy). If you often retune to other tunings (like AEAE) you may find some synthetic core strings like Thomastik will break more frequently.
W.E. Hill dark rosin is a quality rosin that seems to maintain a smooth tone. There are cheaper rosins (that are sometimes free with a violin rental) which I find exaggerate the harshness of the instrument.
Mutes are good to have for practicing, whether you’re avoiding being overheard by others, or you simply want a quieter sound for your own sake. Consider a solid metal bridge mute for maximum sound reduction.
D’addario Micro Violin Tuner is a convenient tuner that clips onto the body of the fiddle and gives a constant easy to read view.
Pano tuner is a free app for your phone that is very accurate and easy to use.
There are free metronome phone apps available as well, I don’t have a current recommendation for a particular one.