We are going to use a simple home budget as a tutorial.  This is not meant to be an exhaustive home budget, but a basic start to a budget to demonstrate some spreadsheet features.

Open a new spreadsheet and save it as "tutorial spreadsheet 1". 
For the below cells in column A, enter the exact text I am telling you to enter, not the amount those might be (that is coming next).
Enter the following in cell A1: Monthly Budget
Note: cell A1 refers to column A row 1.  This is the top left cell on the spreadsheet, right under the A and to the right of 1.  If this doesn't make sense to you yet, it will become clearer to you as you work with spreadsheets more.  Eventually, it will become second nature to you!
In Cell A3 (skip cell A2): Monthly Income
A5: Expenses
A6: Rent
A7: Electric
A8: Heat (Gas/Oil)
A9: Cable
A10: Car Insurance
A11: Car Payment
A12: Gas
A13: Phone
A14: Cell Phone
A15: Total Expenses
Skip A16
A17: Net Per Month

Move the mouse pointer until you are on the line that is in the gray areas between the column headers A and B.  Your cursor should change into a line with two arrows pointing left and right.  If you click and hold the mouse down, you can resize the column by moving the mouse.  In this case, I would like you to doubleclick on that line - it will automatically resize the column to the largest element in the column to the left, in this case column A.

Now, we are going to enter some numerical data in column B.  Some of this will seem obvious, but I'll spell out each item.
In Cell B3: Enter your total monthly income.  For Example "1,800" - without the quotes - would be a legitimate entry.  Don't enter "1800/mo" or anything else with text - we only want numbers in column B!
Skip B4 and B5
B6: Enter your monthly rent
B7: Enter your monthly electric bill
B8: Enter your monthly bill for heating
B9: Enter your monthly cable bill
B10: Enter your monthly car insurance bill
B11: Enter your monthly car payment
B12: Enter your monthly bill for gas for your car
B13: Enter your monthly phone bill
B14: Enter your monthly cell phone bill
Skip B15, B16 and B17 for now

Ok, now we have our basic data in the spreadsheet - let's do some calculations! 

In cell B15 type in (without the quotes) "=sum(B6:B14)".  Note that the letters aren't case sensitive, you could enter "=sum(b6:b14)" or "=SUM(B6:b14)" and it will work the same.  Once you type in the second parenthesis, hit Enter.  B15 should now display the total of your monthly expenses.  Congratulations, you have completed your first spreadsheet formula!

In cell B17 type in "=B3-B15".  Cell B17 should now display the difference between your income and expenses.  Hopefully, this number is positive!

Notice now you can go back and change your income or expenses, and the calculated totals will change automatically - this is one of the many powerful features of spreadsheets!  Once you design your data and calculations, you can change your data and see the results instantly!