Wave function for particle in a infinite well located at -L and +L The wave function for a particle in a infinite potential well located at $0$ and $L$ is $$\psi_n=\sqrt{\frac2L}\sin\frac{n\pi x}L$$ 

I want to find our wave function for the same situation where the walls are located at $-L$ and $+L$

I can write the Schrödinger equation, but I am unable to apply the boundary conditions, so I am unable to find the wave function. 

How can I find the wave function in case of walls at $-L$ and $+L$?
It is very simple, there is no need to solve it again since you already know the solution of infinite square well. The idea is very simple, the trap looks the same, but wider now $2L$. So first you should extend it by a factor of two and then move it back to the center, it is given by the rescaling and translation:
$$L \mapsto 2L \tag{1}$$
$$x \mapsto x-L \tag{2}$$

So the solution is:
$$\psi_{n}=\sqrt{\frac{1}{L}}\sin\left(\frac{n\pi x}{2L}-\frac{n\pi}{2}\right) \tag{3}$$
$$E_{n}=\frac{n^2\pi^2 \hbar^2}{8mL^2}$$
As expected. Note that $\sin(\pi/2-x)=\cos(x)$ so that you can split it into odd and even series, but the Eq (3) given above is more succinct.