You have to learn each verb individually. To us se means myself/yourself/oneself... as an object in accusative while si means to myself/yoursel/oneself... in dative but you must learn each verb separately to know which case it uses.
One complication is that Uč si! does exist, but it means something else. It comes from the non-reflexive učit although that one does not use the dative. I would recommend not to bother about that now.