How can I join a house or make one?

First off, would be deciding exactly What are Houses, Anyway. It may not be the same answer of all purposes. For example a club might have one idea what a House means, a TOS era roleplay could have another, and a TNG/DS9 sim could have a third. I've documented my own concept of what a House is. You may or may not agree with my model, and other models are presented in the first link.

Keeping in mind that this information may need to be adapted depending on your purposes... If you just want a House to claim to be part of for casual Klingon RP then joining a House can be as simple as coming up with a name for it and perhaps filling in a few details of how large and influential the House is, what is the unifying character of the House, who are thier allies, the rivals and their enemies, etc.

In DS9, we are shown one or more 'adoption' rituals, I beleive one was called the Rustai, as Worf, Alexander Rozhenko, Jadzia and Ezri Dax were all adopted into the House of Martok at various points... As a counterpoint in the novel The Final Reflection an orphaned Vrenn is adopted to Thought Admiral Khethas with paperwork and no ritual.

The Final Reflection also gives us the only known model for forming your own House. It seems to be as simple as a declaration that the new House exists. From there, the trick is building and growing it to the point it is relevant.