how do I protect my spreadsheet

Hi Louise, Thank you for your post, welcome to the forum, in answer to your question, protecting the spreadsheet; I am going to answer two questions here, the first is protecting your Workbook against access, and the second is protecting a Worksheet against change.
First: to password protect your Workbook, there are 2 ways, (I know always more than one way to do something in MS Office)
1. When you save your Workbook, in the Dialog box for Save As you will see the tab Tools, click this and choose the option General Options, in the little dialog box that opens insert Passwords as required, (open or modify).
2. To add Password protection to your Workbook whilst you have it open, go to the Menu command, Tools>Options, and choose the Security Tab, the same two choices as above will be available to you.
Second: to protect your Worksheet against change, you would have decided on cells which you "Will Allow" changes to be made in, select these cells/ranges of cells, and go to the Menu command Format>Cells, click the Protection tab and de-select the tick box for Locked, click OK which will return you to your worksheet.
This has now set those cells to be unlocked when you protect the worksheet, allowing users to enter only into these cells and not others.
But before this has come into force you must add one last bit to bring it all together, go to the Menu command Tools>Protection and from the pop out select Protect Sheet, again you will be asked if you wish to apply Protection to locked cells, (remember you have unlocked some) you can apply a password or not, click OK and you will notice that only the areas you wish to allow access to are able to be used without a message box popping up. A good idea now would be to create a Template of this Workbook to be able to use it over and over again. I hope that has helped, best regards Pete.