The first and safest thing to use to clean sensor is a blower.
Then there's something like sensor brush (I havent used this).
Third option is Wet cleaning with eclipse fluid and sensor wand/sensor pad(there are many versions of fluids but the most popular is Eclipse fluid)
Fourth is the new Dust Aid (a pice of double sided sticker stuck on a handle)
Or if youre not that confident to do it on your own, send it to camera repair centre.
Dust in viewfinder will not show up on your images so dont worry about it (though it is irritating, I have some on mine but I only see them when I want to
I bought my sensor wand and eclipse fluid here:
If you google up sensor cleaning, sensor wand/swab, you will be given loads of info about them. A lot of videos are also in youtube.