no thumbnails by posting pictures

Hello!

My website is https://www.janedewaard.nl
and i have ls team twenty ten

i’ve tried multiple solutions to get a thumbnail on my gallery’s.

first method i’ve tried is
“1.new message -> cattogory -> “Portfolio”->
2. menu -> add to menu portofolio ->
3. new message ->make gallery -> “put pictures in the gallary -> give it a title “test”=> catogory “Portfolio”=>Put a picture in thumbnail””=> “public this message”
no result

second method i’ve tried
1. new page => named it “portfolio”
2. add to menu
3. same steps as 3 above here only without putting it in cattorgory
no result

between method 1/2 i did delete all posts/catogories

Im new here to wordpress, but for me it’s really hard to understand it. So if you want to help me please talk to me in childish language with step by step so I can understand it.