Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.
It’s also important to research their payout structure. Not all payouts are the same and tier affiliate marketing often offer different payouts. Some might pay you on the first offer but pay you nothing on any upsells. Ideally, you should be paid for every transaction. In doing your research, you’ll find out which affiliate payout structures are the most profitable.