Sebastian Guadarrama-Sistos , Below is a link to show how to open laptop to access battery for changing. 2nd link shows how you can google search to find a replacement battery using your computers make and model #, link 3&4 are guides telling you symptoms of a bad battery. Good luck.
I hope this helped you out, if so let me know by pressing the helpful button.
https://www.youtube.com/watch?v=XrZYYE1nvmM
https://www.google.ca/search?q=toshiba+satellite+p55w&oq=toshiba+satellite+p55w&aqs=chrome..69i57j69i61l2.2012j0j1&sourceid=chrome&ie=UTF-8#q=toshiba+satellite+p55w+battery&tbm=shop
http://www.howtogeek.com/172362/how-to-know-when-its-time-to-replace-your-laptops-battery/
http://www.dummies.com/computers/pcs/how-to-test-your-laptops-battery/