Query help

Sorry if this is too basic a question for this list, but I don't fully get
how to use aggregates (sum()) and group-by together. I'm trying to get a
list of transactions where the total for a given account exceeds a given
number. I'm not sure an example is needed, but if so, consider this
simplified data:

accountid. name
1 bill
2. james
3 sarah
4 carl

transaction
id. amount. accountid. name
1. 50. 1 bill
2. 25. 2 james
3 35 4 carl
4. 75. 1 bill
5 25. 1 bill
6 50 3 sarah

results wanted-all transactions where account total >= 50

id. amount. accountid. name
1. 50. 1 bill
3. 75. 1 bill
4 25. 1 bill
5 50 3 sarah

I've tried to understand how to use GROUP BY and HAVING, but the penny
won't drop. I keep getting errors saying that all columns in the SELECT
have to also be in the GROUP BY, but nothing I've done seems to produce the
correct results. I think because the GROUP BY contains multiple columns, so
each row is treated as a group. It also is difficult to parse out since in
the real world, many more tables and columns are involved.

Chuck Martin
Avondale Software