A Brahmin is a possible Follower that can carry stuff for you. Depending on what Brahmin you have, it can carry up to 1000kg worth of items. It is useful for Crafting or Delivery Runs.
You have 2 options of getting a Brahmin.
a) You can get an untrained Brahmin by taming a wild Brahmin. You need a rope for that, which will not be used up in the process. This Brahmin will have less Carry Weight then a trained one (Approximately 6 times less), as well as a bit less resistances.
b) You can get a trained Brahmin by buying one from your Faction-Brahmin holder, which resides near the Brahmin Pen in your Faction base. It's price is economy dependant, but expect something in-between 2000 and 8000 faction-currency. This Brahmin has 1000kg Carryweight and more Resistances then the wild one. However, only a limited stock is available! And in order to get more stock you have to sell him your wild Brahmins (for approximately 1000), after which he will train it for 6 hours, then putting it back up for sale. Watch out! Another faction-member can buy this Brahmin out! So if the Pens are empty and you sold a Brahmin for Training, better make an Alarm!
When you own a Brahmin, telling it where to go is as simple as putting Blue RTS-Markers on the ground. Make sure the Brahmin is selected! This can be done by pressing 2x Tab in quick succession (or whatever your Marker bind is), or by holding Tab and dragging a box around the Brahmin.