51% attacks are costly and somebody would do that only if the benefit is bigger than the cost.
As long as your coin is worthless, as long as there are no high value transactions that could be double spent with a 51% attack, you have nothing to fear.

Ideas to prevent 51%?
Invent a completely new algorithm for mining your coin. This way it may not be so easy to get/rent hashing power from other coin for a short time to perform the attack.
Make your coin profitable to mine. Then the miners will come and mine your coin and will make your network more powerful. Yeah, for this the coin has to have success.

Also you can do more thinking and see, maybe you don't need your own blockchain and you can do good enough with a token and the problem of 51% attack is gone, since the parent network is successful enough and safe enough.