First of all, you shouldn't need to patch your kernel just because it's not the latest. You would normally rely on your distribution maintainers to do patching. You might need to patch if you had some kind of uncommon hardware, but most of the time, you just need a different or newer kernel module that is supplied separately from the kernel. My current touchpad has issues with the stock Ubuntu Kernel, but I installed a Package someone built to fix it instead of patching my kernel. Some of the more common reasons to patch a kernel are to test out some new feature or update some more core functionality than a device driver.
Assuming you need or just want to patch your kernel, beware that most patches are against a vanilla kernel as one might download from kernel.org. Most Linux distributions have already applied a number of patches of their choosing against a vanilla kernel which can cause a patch to fail to apply. If you want to learn how to patch a kernel, I would first practice building a vanilla kernel and try to boot from it once before patching it.
To answer your last question, it all depends. I can't tell you whether it's in later kernels without knowing exactly what the patch is and looking at the changelog of 2.6.26. Many people provide patches against the kernel for various reasons. The patch may be a bug fix or maybe just an enhancement. Sometimes, it an experiment to improve disk I/O throughput, or some other experiment. There may or may not be an intention to get it integrated with the next kernel release. A patch does not mean that a bug needs to be fixed, or it may attempt to fix a minor bug, but causing a gaping hole instead and be rejected before ever being integrated.