IMP: how we connect SAP CRM to External system

Hi Experts,

i have a requirement to transfer/upload data between SAP CRM with External System.., actullay i have experiance in ABAP but i never worked on SAP CRM

let me tell how to connect SAP CRM with other External Applicaions(eg.., DOt net)

how can i do..? give me step by step examples scenario to connect with External System (or) we need to Use SAP netweaver XI/PI

plzz help me itz very important....

thankx in advance...