fib.go

package main

import (
	"fmt"
	"math/big"
	"os"
	"strconv"
)

func main() {
	if len(os.Args) != 2 {
		fmt.Printf("Usage: %s <number>\n", os.Args[0])
		os.Exit(1)
	}

	num, err := strconv.ParseUint(os.Args[1], 0, 64)
	if err != nil {
		fmt.Printf("%q is not a valid uint64\n", os.Args[1])
		os.Exit(1)
	}

	p := big.NewInt(0) // p = prev c
	c := big.NewInt(1) // c = prev term
	t := big.NewInt(0) // t = term

	for i := uint64(1); i <= num; i++ {
		t = p.Add(p, c)

		// reset state
		p = c
		c = t
	}
	fmt.Println(t)
}

You can ignore everything before line 21, its just parsing the command line arguments.
also i made the loop start at one, its still looping the same amount though (notice the <=)