Hi Henna,
ABAP is a language of SAP just like C++, Java or any other computer language. ABAP is more of like SQL.
BASIS people take care of authorization roles, system monitoring, installation of SAP system, etc. They are not required to do any coding.
So, to make it simpler for you, if you are interested in writing codes, you should go for ABAP. If you are not, you should go for BASIS.
And also, there are specific functional modules as well in SAP like MM(Materials Mgmt), SD(Sales & Distribution), FI(Finance), HCM(Human Capital Mgmt), etc. in case you are interested in any one of them.
Richa.