Hopefully this website will give you some help:
There is a trail that starts at the Storey Arms. It is a very popular place to start and you can park around there, but you should come prepared with plenty of food and drink, and if you are considering anything more than a leisurely stroll around the base of the mountain you should definitely bring an OS Map and compass and the necessities for surviving on a mountain should anything go wrong - the weather can change very quickly!
This from www.walkingworld.com
Climbing to the highest point in the Brecon Beacons: the summit of Pen-y-Fan (grid ref SO 015215). There are many possible routes to the summit, but the most direct, and one of the easiest, starts and finishes at Pont ar Daf, south of Storey Arms (Walk ID 474). The path itself presents little difficulty but it is strenuous and the summit is prone to mist and bad weather which can disorient you if you're not prepared. You should wear sturdy footwear and take enough warm clothing for cooler conditions on the high ground. Other routes start from the North, nearer Brecon (1304) and from the Neuadd Reservoirs (1290). The high summits can get busy and there are a some quieter, but no less spectacular, walks a little further east (1316, 2617, 559, 2155) – and to the west (2096, 2464)."
Hope this helps - there should be more info on the visitwales.com website too