Get values from other sheet using VBA

Sub TEST()
Dim value1 As String
Dim value2 As String
value1 = ThisWorkbook.Sheets(1).Range("A1").Value 'value from sheet1
value2 = ThisWorkbook.Sheets(2).Range("A1").Value 'value from sheet2
If value1 = value2 Then ThisWorkbook.Sheets(2).Range("L1").Value = value1 'or 2
End Sub

This will compare two sheets cells values and if they match place the value on sheet 2 in column L.

Try

 ThisWorkbook.Sheets("name of sheet 2").Range("A1")

to access a range in sheet 2 independently of where your code is or which sheet is currently active. To make sheet 2 the active sheet, try

 ThisWorkbook.Sheets("name of sheet 2").Activate

If you just need the sum of a row in a different sheet, there is no need for using VBA at all. Enter a formula like this in sheet 1:

=SUM([Name-Of-Sheet2]!A1:D1)

That will be (for you very specific example)

ActiveWorkbook.worksheets("Sheet2").cells(aRow,aCol).Value=someval

OR

someVal=ActiveWorkbook.worksheets("Sheet2").cells(aRow,aCol).Value

So get a F1 click and read about Worksheets collection, which contains Worksheet objects, which in turn has a Cells collection, holding Cell objects...