If you in particular want to donate to Ukrainian lethal military aid, there are several options.
The best option is United24. This is the government's main fundraising platform. They have a specific subsection to direct your donation to defense. Frequently they will also have fundraising events for specific items like naval drones.
Then there is the "Come Back Alive" charity. This is the best non-government way to donate to the Ukrainian military.
Also many individual units will do fundraising for items.