For all primes $p \geq 3,$ define $F(p) = \sum^{\frac{p-1}{2}}_{k=1}k^{120}$ and $f(p) = \frac{1}{2} - \left\{ \frac{F(p)}{p} \right\}$, where $\{x\} = x - [x],$ find the value of $f(p).$
1. We start with the given function \( F(p) = \sum_{k=1}^{\frac{p-1}{2}} k^{120} \). We need to evaluate this sum modulo \( p \).

2. Consider the sum \( 2F(p) = \sum_{k=1}^{\frac{p-1}{2}} (k^{120} + (p-k)^{120}) \). Since \( (p-k) \equiv -k \pmod{p} \), we have:
   \[
   (p-k)^{120} \equiv (-k)^{120} \equiv k^{120} \pmod{p}
   \]
   Therefore,
   \[
   2F(p) \equiv \sum_{k=1}^{\frac{p-1}{2}} (k^{120} + k^{120}) \equiv \sum_{k=1}^{\frac{p-1}{2}} 2k^{120} \equiv 2 \sum_{k=1}^{\frac{p-1}{2}} k^{120} \pmod{p}
   \]
   This simplifies to:
   \[
   2F(p) \equiv \sum_{k=1}^{p-1} k^{120} \pmod{p}
   \]

3. Let \( r \) be a primitive root modulo \( p \). The sequence \( r, r^2, \ldots, r^{p-1} \) is a permutation of \( 1, 2, \ldots, p-1 \) modulo \( p \). Thus,
   \[
   \sum_{k=1}^{p-1} k^{120} \equiv \sum_{i=1}^{p-1} r^{120i} \pmod{p}
   \]

4. We can use the formula for the sum of a geometric series. Since \( r^{120(p-1)} \equiv 1 \pmod{p} \) (because \( r^{p-1} \equiv 1 \pmod{p} \)), we have:
   \[
   \sum_{i=1}^{p-1} r^{120i} = r^{120} \frac{r^{120(p-1)} - 1}{r^{120} - 1} \equiv 0 \pmod{p} \quad \text{if } r^{120} \not\equiv 1 \pmod{p}
   \]
   This implies:
   \[
   \sum_{i=1}^{p-1} r^{120i} \equiv 0 \pmod{p}
   \]

5. Therefore, if \( p-1 \nmid 120 \), then \( r^{120} \not\equiv 1 \pmod{p} \) and:
   \[
   F(p) \equiv 0 \pmod{p}
   \]
   Hence,
   \[
   f(p) = \frac{1}{2} - \left\{ \frac{F(p)}{p} \right\} = \frac{1}{2} - 0 = \frac{1}{2}
   \]

6. If \( p-1 \mid 120 \), then \( r^{120} \equiv 1 \pmod{p} \). In this case:
   \[
   \sum_{i=1}^{p-1} r^{120i} = \sum_{i=1}^{p-1} 1 = p-1
   \]
   Therefore,
   \[
   F(p) = \frac{1}{2} \sum_{k=1}^{p-1} k^{120} \equiv \frac{1}{2} (p-1) \pmod{p}
   \]
   Hence,
   \[
   f(p) = \frac{1}{2} - \left\{ \frac{F(p)}{p} \right\} = \frac{1}{2} - \frac{p-1}{2p} = \frac{1}{2} - \frac{1}{2} + \frac{1}{2p} = \frac{1}{2p}
   \]

The final answer is \( \boxed{\frac{1}{2}} \) if \( p-1 \nmid 120 \) and \( \boxed{\frac{1}{2p}} \) if \( p-1 \mid 120 \).