The only direct influence you have in a representative democracy is your vote. When a politician makes decisions you don't agree with, don't vote for them. However, when a certain issue is very important for you, you can also try to influence the voting decisions of others. In a representative democracy, power depends on public approval, so politicians need to be concerned about those issues which appear important to public opinion.
Use can use your freedom of speech rights to:
- Publicly criticize politicians who make decisions you don't agree with.
- Before the next election, urge politicians and parties to make public statements in matters which are important to you.
- Publicize these statements and urge voters to make their voting decision dependent on these.
- Publicly criticize those who break their promises after election.
You can also use your right to freedom of association to form a non-governmental organization (NGO) concerned with a specific issue and ask as many people as you can to join it. NGOs with many members have more credibility than private people, so their speech can more easily reach a larger audience, which might motivate the media to relay an NGOs statements to their audience.
You can use your freedom of assembly to organize a peaceful(!) public protest against government decisions to call more attention to issues you consider important.
And last but not least, you can run for a public office yourself and promise to make everything better than the current office holder(s).
Warning: This question is about a democratic society. An ideal democratic society permits and protects the rights mentioned above, but not all countries which call themselves democratic are ideal democracies. I do not know much about Brazilian law, so I do not know in which way it limits the freedoms mentioned above. Also note that the human rights situation in Brazil is not very good, which might make it unsafe to exercise your democratic rights, even when within the boundaries of law.