The default will always be AES 128 bit when using BitLocker but you can change this if you'd like that extra bit of encyption strength.
If you bring up a the RUN command (Window+R) and type: gpedit.msc
You will then be presented with: Local Group Policy Editor
- Click on Adminsatative Templates -> Windows Components
- Double click on 'BitLocker Drive Encyption'
- Then to the left click on 'Choose drive encyption method and cipher strength
- Click enable
- Choose AES 256
- And apply
When you have finished the above instructions, insert the drive, password protect the drive and save the key for yourself somewhere secure.
Then next -> choose to encypt the drive
If you dont want to do all this, you can stick with the default 128 bit encyption strength