。
It was tough for me to make the starting lineup.
Literal
I [subject-が] starting-lineup [into-に-なる] [nominalizer-の-は] difficult was.
スタメン is an abbreviation of スターティングメンバー ('starting member'), itself wasei-eigo — not used in English this way. Means 'the starting lineup' in sports. Cleft focus structure: [僕がスタメンになるの]は[難しかった] = '[the thing of my becoming a starter] was difficult.' nominalized clause as topic.