Cu Chi is about 40 km northwest of Ho Chi Minh City centre — around an hour's drive in reasonable traffic, often longer during peak hours.
The easiest way to visit is on a half-day organised tour from the backpacker area around Pham Ngu Lao in District 1. These run daily, cost very little, and include transport, entrance fees, and a guide. Most go to Ben Dinh; if you want Ben Duoc, book specifically for that site.
If you prefer to travel independently, local bus #13 (or the 13B variant) runs from Ben Thanh bus station to Cu Chi town for a few thousand dong. The journey takes around two hours depending on traffic and stops. From Cu Chi town, a xe om (motorbike taxi) or local taxi covers the remaining distance to Ben Dinh.
A Grab taxi or booked private car from HCMC is comfortable and, split between a group of three or four, not expensive. The drive out through the suburbs is unglamorous but the return, knowing what you have seen, carries its own particular weight.