Before investing you have to be clear about your net profit or loss per month.
Profit and loss account
Net income: 2 500 €
Dwelling: 700 €
Nutrition: 300 €
Car rental: 700 €
Car fuel: 300 € (2 000 km x 10 l/100 km x 1.50 €)
Miscellaneous: 250 €
Result
The income (2 500 €) is higher than the expenses (2 250 €). The monthly profit is 250 €.
Investing
With 250 € per month, investing in different assets is possible:
Cash: save at least 3 net incomes (7 500 €) for unexpected expenses.
Stocks: start buying ETFs as soon as your cash reserves exceed 7 500 €.
Commodities: start buying to diversify your portfolio.
Passive income: read this blog post to learn more about passive income.