ᛒᚱᛁᛜ ᛏᛟᚱᛞᛖᚾ. Here's an image file if you don't have a Runic font installed: http://i45.tinypic.com/35ivchs.png
Please make absolutely sure that that is correct before you get the actual tattoo. There are tons of rune charts around on the internet, such as these
, that you can check and double-check with. I don't want to be responsible for a typo or something, hahaha.
Also, I should tell you that this subforum only deals with the modern Norwegian language. So if you want more help with Old Norse, you should post here
instead; they are probably more able to help you.
The file I linked to is only the transliteration of "bring torden" into the futhark, by the way. If you want an actual translation into Old Norse, you should ask around at the subforum I linked to above; I'm not confident enough in Old Norse yet to give you a proper translation, sorry.
Anyway, good luck with your tattoo!