Use one of these exchanges.
Sigscript, not signscript.
All transactions have a sigscript and that is used for proving that the person making the transaction has access to said address, in order to spend some amount of bitcoins.
Witness data is used in transactions involving Segwit addresses as the area to store this proof data instead of in the sigscript itself, since it saves on fees and also prevents a special condition called transaction malleability where you can have two different txids for the same transaction.