Focus on Textbox on Button Click in WPF using MVVM

Question

Hi Good Evening friends,
                               I gone through many articals for getting focus on textbox  on button click.

but i am unable to understand. because there is no tutorial or a perticular blog. in all the post only discussion is going on.
and i am new to wpf and mvvm. i was trying to make a wpf application but after designing form in wpf. i have one new button and in that i want if i click on new button first textbox should be focused. or then if any error occurs due to validation the perticualr textbox should be focused.
so can anybody help me to do that in a easiest way that i can understand easily.
i would be grateful if anybody could help me.


Thanks in advance