The following steps guides you to design ssrs rdl report to add grouping and totals in tablix data region.
To present data in the tabular format, create a dataset and bind data to the tablix data region. In this designing section, the following dataset query is used for dataset creation.
SELECT Top 50 PC.Name AS ProdCat, PS.Name AS SubCat, DATEPART(yy, SOH.OrderDate) AS OrderYear, 'Q' + DATENAME(qq, SOH.OrderDate) AS OrderQtr,SUM(SOD.UnitPrice * SOD.OrderQty) AS Sales
FROM Production.ProductSubcategory PS INNER JOIN
Sales.SalesOrderHeader SOH INNER JOIN
Sales.SalesOrderDetail SOD ON SOH.SalesOrderID = SOD.SalesOrderID INNER JOIN
Production.Product P ON SOD.ProductID = P.ProductID ON PS.ProductSubcategoryID = P.ProductSubcategoryID INNER JOIN
Production.ProductCategory PC ON PS.ProductCategoryID = PC.ProductCategoryID
WHERE (SOH.OrderDate BETWEEN '1/1/2002' AND '12/31/2003')
GROUP BY DATEPART(yy, SOH.OrderDate), PC.Name, PS.Name, 'Q' + DATENAME(qq, SOH.OrderDate), PS.ProductSubcategoryIDRefer Create Data section and create dataset using the above query.
AdventuresWorksdatabase is used here.
Create a simple table report design by following the steps provided in Table Design section.

To add a first row group in the basic tablix structure follow the below steps.
Grouping Panel will be enabled in the design view.
To add first row group in the basic tablix structure, use the group menu in grouping panel.
Row Groups pane in grouping panel and open the context menu on the Details group field.

Parent Group... option under Add Group category.

Parent Group option, a Tablix Group dialog will be opened to configure the grouping.

Product Category field is chosen as parent group in tablix data region. Click on the OK button

Now, a new column will be added to the right side of the detail group in tablix data region and a new group member will be added above the Detail group of Row Groups pane in grouping panel hierarchical view.

Child Group... option under Add Group category.

Tablix Group dialog will open like below.

OK button.

Now, a new column will be added to the right side of the Product Category row group in tablix data region and a new field will be added below of the Product Category group in hierarchical view of Row Groups pane in grouping panel.

In the below steps the total field is created to calculate the sum of sales of each sub category products.
[Sales] expression, and click on the Add Total in the cell menu.

ProdCat group in the data region as shown below.

Now to calculate the sum of yearly sales of each product category follow the below steps.
[Sum(Sales)] expression, and click on the Add Total in the cell menu.

ProdCat group in the data region as shown below.

Now to calculate the grand total of over all product sales follow the below steps.
[Sum(Sales)], and click on the Add Total in the cell menu.

ProdCat group in the data region as shown below.

Modify the each total representation text as shown below.
Total text and set the following expression = "Total yearly sales of " & Fields!SubCat.Value in the expression builder.

Total text and set the following expression = "Total yearly sales of " & Fields!ProdCat.Value in the expression builder.

Total text value from the fifth row, second column cell.

Grand Total. Now the table design will look like below.

Now, the final design will look like below:

In the below design Background color of the total column is set with three different colors . This will differentiate the totals at details level, and product category Level. Set the font weight of the expression fields as Bold to improvise the table design. Also the background color of the tablix header row is changed to improvise the report presentation.

The final report design will look like below:

On report preview, the total yearly sales of each product, product category and grand totals at details group level will be displayed like below.

Navigate to the last page of the report in preview. The grand total details will be displayed at the end of the report like below.

Download the above report design from link.