some tips for a new trader?

If a deal is too good to be true, it is a scam!

If somebody contacts you directly through Steam and says 'you can trust me, look at my +rep on Steamgifts' then that person is almost certainly a scammer and an imposter.

Wait for better suggestions, I'm not the right person for trade advice.
But I can suggest you this browser extension: https://www.enhancedsteam.com/download/ (unless you knew it already).
It'll make it easier to detect scammers, who often impersonate renowned traders from this site, to trick unwary users.

• install steam enanched
• Price is influenced by tons of little things: lowest price available now (and historical lowest price), desirability, 4-packs available or not, bundled or not, how many times in bundle, etc...

https://www.reddit.com/r/Steam/wiki/scamtypes

You might want to check out this thread: http://steamcommunity.com/groups/bartervg/discussions/0/351660338682826769/

For the most part, I've been pleased with my experience on barter.vg

Don't do it. Not worth the hassle you'll inevitably run into numerous times.

trade with people with high rep
check if you are trading with the real user (compare the profile here and the one you are talking to on steam)

Although they specifical deal with Steam trading, SteamRep has useful information in the SteamRep Guides subforums. One such is a good general overview.

Remember, you are trading with a Steam ID, not a name. Always use the Steam ID when checking on a trade partner and always use it to initiate communications.

Never let yourself be rushed. Rushing leads to mistakes in procedure and judgement.

If something looks too good to be true, it probably is.

conversion game<=> csgo/dota/tf key

I know that CS:GO case key is valued at 2$ by traders. Since whenever Anomaly/McSkillet/whoever else mentions the price in keys and then in $, price in $ is always double the keys

Use Steamrep to check your traders.
SteamRep Pre-Trade Checklist

Also +rep on someone's Steam profile is more than useless as it can be so easily faked and abused.

If you're trading in keys you're likely going to be paying the equivalent of the game's price or more. Check to see if the game was bundled.

If you're just trading games then people value them differently, so it's up to the individual.

use barter.vg for trading, it's awesome
always check Steamgifts REP and barter.vg REP and whatever REP source is relevant. REP comments on Steam can easily be falsified.

Steam enhancer plugin or something like that is quite useful as it also links to such relevant sites on ones profile

Not worth it, between people trying to scam you and endless low ball offers to you.

If someone adds you on steam without saying anything, always ask for his steamgifts profile and use the link on the profile to make sure you're dealing with the right person (and use enhanced steam as mentioned above). Also, if a deal seems too good to be true, then it usually is. Just be really carefull and take a look around the forum for thread from people that got scammed to find out what people do to scam. Knowing the scamming tactics helps a lot (and if you do get contacted by a scammer and you realize that, you can totally have some fun with him :p).