I wnt to get from Hammersmith to Great Portland Street on the Underground, my nearest station seems to be Ravenscourt Park - Do I need to change anywhere ?
you can go from Hammersmith to Great Portland St on the Hammersmith & City line (the pink one) without changing, but if you want to go from Ravenscourt Park on the District Line you'll have to change at Hammersmith. I think the Hammersmith stations on the District and H&C lines are actually separate ones.
You may be better getting the District Line to Edgware Road (changing at Earls Court if there are no through trains) and then the H&C or Circle line on to Great Portland Street. No need to change stations at Hammersmith if you do this.
Hope this link of the two other stations in the vacinity might help make your life a bit easier. I use Great Portland Street when I go to Harley Street, Regents Park is equidistant but might save you some time if you are going to Marylebone Village or the Medical Zone. Hope this helps:
when you buy your ticket request a free pocket size underground map and keep it with you, this should give you more confidence, also most people[ other than tourists] are very helpful if you ask.