First of all if you are using Excel it needs to be Excel Online for the connector to work properly. Once you have that confirmed or the file moved into Excel Online continue as detailed below.
On the lefthand side of your screen you will see the different menu options. Tree View, a plus icon, then a cylinder icon that means data sources, click the data source icon. This should pull up a menu called 'Data'. There should be an Add Data button that you need to click. Once you click that type Excel Online and then select Excel Online (Business) from the list. This will start the data source linking process which will ask you to point it to the file and table you are wanting to add to the app. Once done you should see the excel table listed as a data source in your Data menu.
Now that the excel table is linked to the power app lets talk about the gallery issues. Galleries in Power Apps are very easy to work with as you will learn. If your data source has one item or 1000 items, you will only have to ever edit one to edit them all. This is where the comment "If I click on the first box every box changes" comes into play. What you want to do is add labels and other controls that will store the column data for each item. You set up the first item and then the gallery applies those settings to each item in the gallery for you after that. Here is an example where I have a gallery that displays user's time clock entries. There are 5 fields shown in this gallery. StartTime, EndTime, AuctionName, AuctionCategory, and HoursWorked. These will be filled out for each item using this data from your data source.
Let me know how this goes as it is kind of hard to describe. If you want more help please ask! 🙂
