Between London Euston and London Victoria train stations?

On Monday I am going from Liverpool to Sittingbourne to collect a new car, according to Virgin Trains I have to change at Euston and get on at Victoria to get to Sittingbourne, but the last time I went to London I was about 14 so dont remember and dont want to get lost either.

Could someone tell me where I need to go when I get to Euston so I dont get lost or mugged .

Cheers.