How can I get past this plateau in my workout routine?

Ok, I have been doing push ups for a month and half, but I don’t gain any strength to do more.
I do 15 sets of 10 reps, and from today on I will do 15 sets of 12 reps.
I do millitary push up for 3 sets and I struggle, then the rest of the sets I do them with my knees.
In the 10 set I usually get really tired, and I attach my feets to something, so I can push myself.

If you have any advice to how to become stronger and better at push ups, would be appreciated.
Thanks

Ps: I’m a beginner