As has already been stated, the Underground station you're aiming for is Tower Hill. (Then follow the signs to Fenchurch Street station. It's just a few minutes walk).
As I assume that you're arriving at Euston by train, your rail ticket should include travel by Underground if it's a through ticket to a destination served from Fenchurch Street. (To check look for a small 'cross' symbol on your rail ticket. Depending upon the type of printer used, it either looks like + or like �. If that symbol is present, you can use your ticket between Euston and Tower Hill, without buying a separate Underground ticket).
At Euston station, look for signs for the Northern Line. (It's the black one on the map). Unfortunately, the Northern Line can be a little confusing because it has two branches. Take care to check that you board a train routed 'via Kings Cross - St Pancras'. (It might also be signed as 'via Bank'). Travel for 4 stops and alight at Moorgate.
At Moorgate, look for signs for the Circle LIne - Eastbound. (The Circle Line is yellow on the map). Travel for 3 stops to Tower Hill.
Don't worry. It's really easy and (although they're much-maligned) London Underground staff are very helpful.
Chris