So at the basic level what you can do is start with the YouTube search bar. When you type something in it will give you suggestions, and these are usually similar things other people are searching for. I would take the most basic keyword you can think of for your topic and type that in, and see what it suggests. Then you can go through adding different letters after that and see lots of different possible keywords.
What you want to find are keywords with the right ratio of demand vs competition. *Usually* these are going to be 3 or 4 words long for a smaller channel. Depends on the amount of competition for your topics though.
I personally use a tool called morningfame that helps to give some letter grades to visualize the possible demand/competition. However if you want to do it yourself, most of the terms suggested in the searchbar are to my knowledge being suggested because YouTube thinks there is demand for them so that can be a good indicator for that. You can also search for content on the keyword and see how similar content recently has performed. Or you can use outside tools like steam charts for video games to judge how many people are playing games and what may be trending. For competition I would search the term and see how many other people have videos using that exact search term in the title/tags. Sometimes looking at small creators can be good to judge a good ratio since if there is a lot more demand than competition it will overflow to smaller channels.
To optimize, it's really 3 main things, you want the keyword in your title, description, and tags. Morningfame has also indicated to me that using related keywords that use words in the main keyword in both the description and tags can also help make your video extremely relevant to that keyword.