I would like to post an exercise that I am practicing to strengthen my fingers, I hope this helps a beginner. I copied this from my lesson+theory+technic piano course book.
While FOUR fingers hold the whole notes down, ONE finger plays the quarter notes. Repeat each measure may times.
1. In the first measure, press down the five keys together (GABCD, all the keys in G position), then raise the thumb JUST HIGH ENOUGH TO LET THE KEY RISE TO ITS LEVEL, keeping the thumb in touch with it. Now have the thumb press the key down again, hold it at a moment, then rise again.
2. Continue in the same manner with 2nd finger, raising it about one-third of an inch and striking the key repeatedly while the other fingers hold their keys.
3. Proceed similarly with the 3rd finger, keeping the others down.
4. Now continue with the 4th finger, but raise it AS HIGH AS POSSIBLE, so that this hampered finger may gain more independence.
5. Continue with the 5th finger, raising it also AS HIGH AS POSSIBLE, so that it may acquire more strength.
Keep fingers curved at all times.
