Use this bot to get ID for different entities across Telegram:
• Forward message from channel to get channel ID;
• Forward message from user to get their ID (unless they restrict from doing so);
• Send a sticker to get its file_id (currently you can use the sticker's file_id with any bot);
• Add bot to group to get its ID (it will even tell you when you migrate from group to supergroup);
• Use inline mode to send your Telegram ID to any chat.
Source code:
https://github.com/MasterGroosha/my-id-bot.