Do I need to sign 'I' before signing 'like'?
Not necessarily! Because the sign for 'LIKE' starts directly on your own chest, it often inherently means 'I like' in a first-person context. You can add the sign for 'I' (pointing to yourself) for emphasis, but it is not strictly required.
Is this the same sign used for 'similar to'?
No, this sign is exclusively for expressing fondness or preference. If you want to say two things are alike or similar, you would use the 'Y' handshape moving back and forth between the items.
How do I sign 'don't like'?
To sign 'don't like', you start the exact same way as 'LIKE', but as you pull your hand away, you flick your fingers outward and turn your palm down, as if throwing the item away. You should also shake your head to indicate the negative.