As stated before, the double bond in alkenes is a source of electrons. Alkenes are weak bases because the π-electrons are only available after breaking the π-bond first. Nonetheless, alkenes are capable of becoming protonated by strong acids. The most important principle to keep in mind is that with unsymmetrical alkenes, formation of the most stable carbocation is the preferred outcome.
Protonation of the unsymmetrical alkene favors formation of the most stable carbocation. In this example the tertiary cation is favored over the secondary one.
Can you show the electron movement taking place here and the exact location of the new C-H bond in the carbocation that forms?