The Golden Mount Temple is located in the centre of the Old Town, in the Pom Prap Sattru Phai district.
The Sam Yot metro station on the MTR Blue line is the closest one to the temple. From the station, it's a 10-15 min walk to the temple. You can also take the BTS Skytrain and get off at the National Stadium station, which then includes a 30 min walk.
Depending on where you are coming from, you could also take a ferry across the Chao Phraya River to the Phanfa Pier station. The temple is a short 5 min walk from the pier.
You could also take a taxi or tuk tuk to the temple complex.