Try to put a lot of tags in your videos!If you can't then you have a problem...But,I know what you are feeling..I had the same problem!
But with this little tips you can easily find Tags for your videos!

1.Use Google Adwords.
You basically search there for words that have to do with the theme of your video (If you do a Pokemon Video and you type in Google Adwords ''Pokemon'' you will most likely get top-searched words like: Pikachu,Pokedex,Charizard,Pokemon X and Y,Kanto etc.)

2.''Steal'' from others.
You don't basically steal from others!(REMINDER:YOU DON'T STEAL)You just get an idea of what to use.You search in Youtube for let's say Minecraft Gameplay/Mod videos because you upload a Minecraft Video and you want tags.You will go to video that you think is really famous,go to the video and while you are watching the video,in the white area press right click and then ''View Page source''.From there in the top of the page you are going to find the Keywords section (Basically tags) and you can easily copy-paste them or just get an idea and the go and put your own tags!

Thanks for reading this and I hope I helped!