1. A wallet should: give you control over your private keys and need to be open source.
2. An exchange is meant for buying/selling crypto and not for storing your funds, it doesn't matter If you have 2FA enabled or not, your funds are not secure there because you don't have control over them. (Think of the following scenarios: Hacks, inside jobs, exit scam...) So just trade and withdraw to your personal wallet.
3.In general, a reputable exchange will be listed in CoinMarketCap and has a few millions in trading volume, don't try to use something else. If you find an exchange that has a lot of advantages and you have some doubts about, feel free to ask in the forums.
4. If you're into ICOs, avoid projects with hype around them, only invest in something that you believe in.
5. Avoid cloud mining companies and sites that promise to give daily interest, It's a Ponzi scheme.
6. If you're dealing with other individuals, avoid accepting reversible PayPal payments unless that user is trustworthy. If dealing with someone in the forums, watch the profile and see If there was a recent password change (account hacked), and also the trust history.
7. A little bit off topic but you should avoid post bursting.

I'm not sure how deep you are into crypto and If you already know about this or not but hopefully these pieces of advice could help you and save you some money.