how can i choose my profile picture in word press websites? i cant find a platform to upload my profile picture in one of websites that i am join it
Most "profile pictures" as you call it (if you're talking about in comments section, for instance) are pulled from whatever comments system us being used. The built in system use Gravatar, and other systems, like Facebook, Discussion, Twitter etc uses the profile pictures from those networks. If you're talking about posting something, WordPress doesn't have a default "profile picture" - for that you'll need some sort of plugin.
If you are talking about uploading profile picture on self-host wordpress, then you can install the wordpress plugin called simple-local-avatars.
The login area might be restricted by the admin. You can contact the admin and ask him/her to give you the access.