In the quantization of the open string for example, we are told that the state space is
$$|p \rangle$$
$$\alpha^i_{-1}|p \rangle$$
$$\alpha^i_{-1}\alpha^j_{-1}|p \rangle , \ \ \alpha^i_{-2}|p \rangle$$
and so on...
and, off course, we cannot forget that we have to impose the virasoro constraints:
$$L_n|\psi \rangle=0 \ \ n>0$$ $$(L_0-1)|\psi \rangle=0$$
There are some problems with the tachyon but I would like to ask that in a separate question, so let's see the next state $\alpha^i_{-1}|p \rangle$. I know that $p^2=0$ must hold for this state, but is there any restriction for the value of $p_0$? Are there $p_0<0$ states in the spectrum? Are they banning the negative-energy solutions by hand?