Step 1: Overview.
Acid-catalyzed dehydration of ethanol to ethene proceeds in three steps using concentrated $H_2SO_4$ at high temperature.
Step 2: Step 1 Protonation of $-OH$.
$CH_3CH_2OH + H^+ \rightarrow CH_3CH_2\overset{+}{O}H_2$ (oxonium ion). The $-OH$ is converted to $-OH_2^+$, an excellent leaving group.
Step 3: Step 2 Loss of water.
$CH_3CH_2\overset{+}{O}H_2 \rightarrow CH_3\overset{+}{C}H_2 + H_2O$. Water leaves, forming a primary carbocation. Heat drives this endothermic step.
Step 4: Step 3 Loss of proton to form $\pi$ bond.
A base (water molecule) abstracts a $\beta$-hydrogen: $CH_3\overset{+}{C}H_2 + H_2O \rightarrow CH_2=CH_2 + H_3O^+$. The electrons form the $C=C$ $\pi$ bond, giving ethene. The acid catalyst is regenerated.