This is a basic form of the ancient game of Nim.

You will need seven objects, such as counters or blocks. It is a game for two players.

Place the 7 counters in a pile and decide who will go first. (In the next game, the other player will have the first turn.)
Each player takes turns to take away either one or two counters.
The player who takes the last counter wins.

Keep playing until you work out a winning strategy.
Does it matter who has the first turn?
What happens when you start the game with more counters?

There are more Nim-like games here .