Two places spring to mind - Monknash in the Vale. Not easy to get to as it involves clambering over a lot of rocks and you have to time it right for the tides otherwise you can be cut off. Perhaps better for a first timer is Kenfig between Porthcawl and Port Talbot. A walk of about 1.5 miles from the car park at the nature reserve, not too busy and plenty of dips in the dunes to afford some seclusion whilst you build up your confidence. Further afield are a couple of beaches on the Gower though I've not personally tried them but heard good reports and if you want to take the plunge and get straight into going nude in public then Tythings Barn near Kilgetty in Pembrokeshire is a fantastic naturist campsite that's also open to day visitors extending to around 30acres of lakes, woodland and foreshore on the upper Milford Haven. Contact me if you want more info or moral support.