Hi there and welcome to aditnow.
Your last sentence says it, "no experience". Recommend that you join a club to gain this. Recommend Shropshire Caving and Mining Club, but hold back with any activities until the current public health crisis is passed.
The mines that you mention have been accessed, but by vertical rope techniques which require a high skill level. Lefel Fawr does go for some distance and undercuts the mines, but little of the workings are actually accessible.
There should be some material on here inc photos etc. It was on the meets list for the 2019 NAMHO conference, more on this site.
http://cambrianmines.co.uk/NAMHO_2019/pont.html I would urge you strongly to keep out of the mines unless you are prepared to spend money on personal safety gear, helmet with chin strap with fixed electric light as bare minimum, and then only explore in the company of others who have previous experience.
My avatar is a poor likeness.