Sure, sign the petition but don't donate. Change.org is a for-profit, "venture-backed company that hosts activist petitions written by members of the public, gathers email addresses from signees, and encourages people to circulate the petitions heavily on social media.
If you wish to make a donation to relief in Ukraine try organization where at least 90% of the money goes to helping those in need. Such as:
https://www.crs.org/
https://www.clevelandmaidan.info/
I personally have given to both of these organization and have checked them out thoroughly. Both charities are 501(c)(3) non-profit and fully tax deductible with over 90% of the donations being used to help people in need. Cleveland Maidan is 100% volunteer and on their own nickel some of the volunteers are in Poland at this moment helping refugees.
Full disclosure, both sets of my grandparents came from Ukraine.