If you're playing Black White 1, then it is not so simple. In fact, it so complicated, that I suggest trading for one.
However, since your tags indicate B/W 1, these are the Pokemon you must have:
- Cacnea (Through Poketransfer or Dream World)
- Breloom / Shroomish (Found in Black only)
- Dwebble or its evolution, Crustle
If you have all those, first, breed a Male Cacnea with Spikes (through level up) with a female Dwebble.
The resultant Dwebble will know spikes Now breed this (male) Dwebble after it learns Stealth Rock at Level 28, with a female Ferroseed to get a Ferroseed with both the moves.
Finally, breed the Ferroseed with a male Breloom (with Seed Bomb, learnt at level 41) to obtain desired Ferroseed, and then Ferrothorn.
In Black/ White 2, it is much simpler.
Just Breed a Ferrothorn (Female) with a male Cacnea knowing Spikes.
Teach the resultant Pokemon Seed Bomb and Stealth Rock at the Move Tutor (s) after saving up Shards.
Sources 1 and 2.
[Credits to Mew for for the first bit, as seen in the source.]
