You are making progress. That is very good.
Here is what I enter into Mathematica
In[1]:= f[x_] := x + 1
In[2]:= f[2]
Out[2]= 3
In[3]:= f[a]
Out[3]= 1 + a
The In[ number ] := and Out[ number] = are supplied by Mathematica and I do not type those.
At the end of each line I press and hold Shift while pressing Enter. That tells Mathematica to do the work for that line. For the first line that just tells Mathematica to remember your definition and it displays nothing. For the second line you are asking it to calculate f[ 2 ] and it prints the third line of 3. The next two lines are similar.
I will try to attach a notebook that you may be able to download and read into Mathematica by double clicking on it or by opening Mathematica and then clicking File and then Open... and selecting the file from where it was downloaded to your computer.
If you have more questions then please ask and I will answer later today.
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