Many Germans are used to - and highly appreciate - native English speakers who try their hand on German words and phrases. They are aware of the idiosyncrasies and difficulties English speakers encounter when trying to get their tongue around particular German words and sounds like the umlaut and the 'ch'. Although you should try to get it right, don't be too concerned about not being understood.
The five basic vowels are either pronounced short or long, depending on the word.
A = sounds like the u in 'fun'.
E = sounds like the ay in 'day'.
I = sounds like the ee in 'see'
O = sounds like the o in 'over'
U = sounds like the ou in 'you'.
Ä ä with 2 dots is an Umlaut and is also correctly transcribed as ae. It sounds similar to the a in 'man' or like 'bed'.
Ö ö with 2 dots, also transcribed as oe, is tricky and a sound which does not exist in the English language. The closest would be the ir in 'sir' but with a mute 'r'.
Ü ü with 2 dots, or transcribed as ue, equally does not exist in English. It's pronounced like the French word 'muse'.
'ch' as in the German word for "I" = ich, sounds like the h in 'human'.
'ch' as in the German word for "small river" = Bach sounds much harder and is pronounced like the Scottish word 'loch'.
'ei' as in the German word for 'near" = bei, sounds like the i in 'while'.
ß or 'ss' is a sharp, short s, pronounced like the sc in 'scissors'.