If you look up ´only´ you´ll find a.o. the words yalnýz and sadece,

but also ´bir´ can be used (bir seni seviyorum = i love only you)

Any rules as when to use what?

I´m not 100% sure but I would use sadece where the meaning of only is also ´just´ as in ´I only want to speak to you´ (I just want......)Sadece seninle konusmak istiyorum

Then yalniz when it means only as in ´only one´ or even ´alone´  as in ´my mom has only one eye´

Others can probably offer a better explanation.

You can use "bir" or "bir tek" to emphesize something emotional generally, before the word or the word group which you want to emphesize. And that word or the word group must be before the verb;

Ankara´ya bir tek senin için geleceðim

Bu konuda bir ben yanýldým

Yüzlerce þarkýnýn arasýnda bir bunu seviyorum

"yalnýz" comes from "yalýn"

"sadece" comes from "sade"

"sade" and "yalýn" are synonim words.

Örnekler: (as a native speaker)

* Sadece seni sevdim. = Yalnýz seni sevdim = Yalnýzca seni sevdim. (They are all sounds same to me)

* Yalnýz öldü ("He/she died alone" - Without no one) 

* Sadece öldü (Means "he/she died - ´it´s just a death´