How To Find KW For YouTube Videos...

You can try using the Keyword Suggestion Tool by Youtube.
https://www.youtube.com/keyword_tool

@Weblin - it seems that this feature is no longer available for URL or video ID keyword generation.

@Moneyland - start your video optimization with a keyword research session. Looking at other videos and copying their keywords is not enough to outrank them in video listings. You need to create and add your own set of keywords, specific for the video content you're creating.
There are multiple keyword tools you can use (paid and free). The most common one is Google Planner, a free keyword tool created by Google (here you can read something about using it - http://www.advancedwebranking.com/blog/keyword-research-with-google-keyword-planner/)

You need to create a free Google account in order to get keyword ideas.

If you want your video to generate loads of traffic, adding keyword tags is sometimes not enough. Here are some other simple strategies you can consider:

- Transcribe the video and add the entire text as a description - video and image content still can't be crawled by search engines, so adding a descriptive text is useful

- Link to the videos from your videos and use all the linking resources you have - social share it, comment with it on relevant blogs and forums etc.

- Add a relevant title to your video with keywords included - here you should place your primary keywords, the ones you struggle to rank for.

- Find other popular videos in your niche. Youtube allows posting comments with videos - do this with a link pointing to yours and you could also receive traffic.

Regards and good luck!

Great response @Adam. Very useful info on researching keywords.
and ranking keywords
@Moneyland you can still find the keywords on the view source page. They can be a bit hard to find but if you do a search for the word "keywords" on the page you will find them.

There are some people on Fiverr that do keyword research that might be able to help you