Metronome: http://www.metronomeonline.com/. Click on "A440" to hear an A, to help you tune. Click on different dots around the dial to choose a speed for the beat you want to play to.
You can also just Google "online metronome" and one should automatically be there at the top of the search.
Tuners: These will help tune your open strings when practicing at home. Remember, only use the fine tuners until you have a lesson from me on how to use the tuning pegs.
Violin Tuner: http://www.violinonline.com/tuning.htm
Viola Tuner: http://www.violaonline.com/tuning-viola.htm
Cello Tuner: http://www.celloonline.com/tuning-cello.htm
Double Bass Tuner: http://www.stringbassonline.com/tuning.htm
or
https://www.tunestrings.com/ - search under the Instruments menu (I couldn't find Double Bass, however; maybe you can! Electric Bass will work fine)