Hi. I am looking for some cheap accommodation in Swansea for one person, about 15-20 quid, if it is possible, for one month in autumn. Could you advise me, please, where can I find it? I have looked for it everywhere in internet, but I have not found it yet. Thank you very much.

Anywhere selling accomodation for £15 to £20 a night would not advertise on the Internet as prices normally start from around £25 or £30 a night.
Youth hostels are your best bet but these usually allow you to stay for a short time, not a month.
Telephone the tourist office in Swansea and see if they can help. Failing that, when you arrive in Swansea, take a walk down the sea front road (Mumbles Rd) where there are many guest houses and small hotels.