It is important to have a metronome that subdivides the beat. There are plenty available from local music stores but there are some mobile app based ones I also like. I've listed iOS apps because that is what I'm familiar with. Some of these are also available for other platforms
SpeakBeat (this metronome actually has a spoken voice, very useful)
I use a clip-on tuner that is very durable, extremely accurate and can be used without interference from other instruments in the room. It is the Snark SN-8 and I highly recommend it. There are some mobile app based ones I also like. I've listed iOS apps because that is what I'm familiar with. Some of these are also available for other platforms
Having fresh rosin is very important! The two brands that I use are POPS and Kolstein Ultra Soft. If you use POPS it is important to replace it every couple of months because it dries out quickly.
Here are resources for MIDI files and a player to play those files on mobile. Note that the MIDI files on the Michael Wolf website are free but password protected. Prof. Wolf asks that you email to request use of the files. The instructions are on the website.