Here are a few answers. I'm not going to try to answer them all at once.
"Verticaly I have, for example, merged a few cells and called the cell "Services"."
It a poor practice in Numbers to either Merge or Split cells. It causes problems later if you decide to sort.
"I am making a spreadsheet of my month by month costs, with Months across the top"
It's generally best to have expanding data running down the page, rather than across. If you are going to add months later, put the months in a column rather than a row.
"(I am not sure when a top row becomes a Header)"
A top row, or rows, gain the Header format attribute when you set the number of Headers in the Table Inspector. Try it.
"how can I attach 'hidden' notes to any cost as a future reference"
In general, subtotals and totals belong in Footer Rows or separate Summary Tables.
I made a document for my budget and used many sheets-- one sheet for Utilities, one for Income, one for Car, one for House, one for Charity, etc.
I also had a summary sheet which took information from all those sheets and presented the main catagories.
As far as future notes: there are a couple ways...
1) attach a note to the cell or
2) enter a formula show how you arrived at the total. That is if the total is comprised of adding 120 and 80 and 35 type "=120+80+35" in the cell
I would not merge any cells as generally that causes problems later
You make rows headers of footers at your discretion. To make a row a header, click the menu for that row and select "Convert to Header Row":
And don't forget there is a Users' Guide which can be helpful:
Please post back as needed. There are many people willing to help.