How to find seasonal effect and trend from raw data (newbie ...)

Hello,

From the data provided in the attached file, I would like to find the

seasonal effect and the trend (if there are some) like in page 10 of the
following document:
http://www.statoek.wiso.uni-goettingen.de/veranstaltungen/zeitreihen/sommer03/ts_r_intro.pd
http://www.statoek.wiso.uni-goettingen.de/veranstaltungen/zeitreihen/sommer03/ts_r_intro.pd
f

Small problem: I don't know how to start ... 
If someone could just explain me how to convert the data in a way I can use
stl (or another function ?), it will be very helpful to me.

Thanlks in advance,
Ptit BLeu.

http://r.789695.n4.nabble.com/file/n3936136/Date_Y1.txt Date_Y1.txt 
To see the data:
aaa<-read.table("Date_Y1.txt", sep=";", dec=".", header=T, as.is=T)
plot(strptime(aaa$Date, format="%y-%m-%d %H:%M:%S"), aaa$Y1)



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