If you have just put adsense on your site give it time. Google usualy take some to deliever targeted ads.
You could use section targeting to tell adsense what text to look at when deciding what ads to give you, Though this usually takes a week or two to take effect.
From my experience with this, I think you can put some text (that's related to your content) around your adsense code and wait a little bit for Google. The ads will change according to the phrases you put.
Yeah usually when you first stick the ads on they are some random ad. If it's a general forum or website, you will get random advertisements most likely.
If it is a new site, dont do anything. The system will bring relevant ads soon.If it is an old site, then either there are no advertisers for the keyword you are using or you have diluted the keywords so the bots are confused.
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