There are many places online to find genealogy info; these are some of my favourites.
Ancestry is a world-wide subscription site, (my sub is UK only), with all sorts of useful things - like indexed census info up to 1901, BMDs, christenings, marriages, burials, probate details .... to name but a few. Most UK libraries have a subscription to the world-wide site which you can use.
FindMyPast is a UK variation on Ancestry with the added advantage that they have the 1911 census. Various subscriptions are available. I currently have the 1911-census-only version.
FreeBMD does what it says on the tin. Its indexes have been transcribed by volunteers.
FamilySearch is the updated version of the IGI. Some massive improvements here and, like FreeBMD, it doesn't cost anything.
IGI Batch Numbers, put together by Hugh Wallis, enables you to search for christenings on the IGI by location.
Collections Canada for any Canadians in the Great War
Australia Trove gives access to Australian newspapers
The Commonwealth War Graves Commission is a must on your "favourites" list
The British Newspaper Archive is my latest favourite
I don't have many relatives up there but, if you have some in Scotland then Scotland's People is the place to go.
And loads more ....
AND SOME OTHER SITES I'VE BEEN TO:
Bob Hutchins' site for the Hutchins/Westwood families is well worth a look, particularly at his Stretham collection.
For all your old postcard needs, you might as well look at Old Postcards first .... superb service, excellent cards.
I'll keep adding them as I find them ....