I bought a new talis teh other day and this naturally led to the question of whether you say a shehechiyanu and when do you say it.
The first issue is whether you say a shehechiyanu on a mitzva. The Rambam (berachos perek 11) lists 3 catagories of mitzvos where you say a shehechiyanu 1) mitzva done m'zman l'zman like sukkah and lulav b) mitzva that you acquire like tallis and tefilin c) mitzvah that is not tadir like pidyon haben and mila. (I am not sure exactly what the lomdus of the Rambam is). Tosafos (Sukkah 46A) writes that you need two requirements for a shehechiyanu on a mitzva. It has to be chashuv and bring you simchah and it has to be m'zman l'zman. Therefore, you don't say shehechiyanu on a new tallis or tefilin. (Again I am not sure what the lomdus in the machlokes is)
The Mechabeir paskens like Tosafos but he adds that since a talis is a new garment you can say shehechiyanu on the new talis for that reason. However, you would not say shehechiyanu on tefilin. The Magen Avraham agrees with this and the Taz is choleik and holds you would say shehechiyanu on tefilin (like the Rambam). Interestingly, although the EMchabeir holds like Tosafos, Tosafos himself rejects the idea that you say shehechiyanu on a talis because it is a new garment. According to Tosafos, you need the additional requirement of being m'zman l'zman.
As far as when to make the beracha, the Rema adds that you should make it when you put on the tzitzis. The Magen Avraham explains that really the shehechiyanu on a new beged should be made when you buy it. However, you don't make a shehechiyanu when you buy a beged which can't be worn right away. So if it needs to be tailored you make the shehechiyanu when you wear the beged (ayin Siman 223:4). Therefore, by a talis you can't wear the beged until the tzitzis are put on. Really the beracha should be made the first time you wear it. However, in order to be choshesh for the Rambam we make the shehechiyanu when we do the mitzvah of putting on the tzitzis.
Nireh Li, that l'ma'aseh it seems if one buys a talis in the store that has tzitzis then the shehechiyanu shoould be made when you buy it. If one didn't make the beracha at the time you bought it, it is clear you can make it when you wear it. The Pri Migadim actually says you can make it even after wearing it teh 2nd or 3rd time.
Finally, assuming you didn't make the beracha until you wear it, which beracha comes first, shehechiyanu or l'hisateif b'tzitzis? This is a machlokes haposkim. Teh Be'er Heitev quotes the Olas Tamid who says you make a shehechiyanu first. The Mishna Berura quotes the Pri Migadim as saying you make l'hisateif first. The Sha'arei Aharon points out that the problem with the Mishna Berura is that the Pri Migadim doesn't actually say this. In fact the Pri Migadim seems to imply you say the shehechiyanu first since he compares it to eating a new fruit and by the new fruit the Pri Migadim says to say shehechiyanu and then the berach aon the fruit. Furthermore, the Mishan Berura himself paskens by a new fruit to say shehechiyanu first. It is not clear why here would be any different. I think it could depend on whether you view the shehechiyanu as a hefsek but tzarich iyun on the Mishan Berura.