Under the gun is the toughest position to play NLHE. This is why it is so important to know how to select your range when you are sitting under the gun.
Under the gun hand selection really doesn't vary much from a 9 handed table to a 6 handed table.
Some players who are creative with their plays will open a much wider range of hands up, but the players who will be in the tournament for a longer period of time, or maintain a chip stack will play tighter. At a 9 handed poker table the weakest hand that you want to open the pot with under the gun is AQ off suit, and a hand like 99.
Most players limp into the pot with middle pairs or hands that don't beat JJ if they are playing from under the gun. You are better off limping into pots early in the tournament when blinds are small in the tournament when blinds are big.
If you have small pairs sometimes it is alright to limp into a hand if your stack size is big enough where it doesn't affect it enough to make a difference.
At a 6 max table it gets tricky as to what hands to open with, but at worst AJ off suit or any pair. The game is changing quite a bit, so opening with any pair for a pot sized raise in a 6 max game is a good way to go. If you get three bet before the flop, it is tough to play small pairs however.
You will be playing out of position and praying that you spike a set. A lot of the time it is better to just fold to a re-raise before the flop and invest nothing else in the pot. If another player calls the re-raise though you are now getting the correct price to hit your set on the flop.
You can burn a lot of money by checking and folding flops with small pairs which is why the check raise is a great play to make on the flop if your opponent three bet.
The reason is because a lot of the time they will have AK. If no ace or king comes on the flop most of the time you can check raise and steal the pot away from your opponent.
Playing 6 max games you can certainly try out a wider range of hands, but if you do the variance also goes up. If you choose to open the pot with a QJ from the gun and get 2 callers with a jack high flop you could get yourself into some trouble.
It is left up for interpretation of what you should do because it depends on how your table is playing. A 6 max game can be very swingy simply because players are more aggressive than a 9 handed or full ring table.
Playing from the gun there are two things to keep in mind; can I call a three bet, and how much value can I get for my hand. If you cannot call a three bet with your hand then you should be laying your hand down before the flop.
If you can get value from your big pair and can call a three bet then you should be moving your money in and seeing the hand through.
The online poker room with the largest choice of ring games and tournaments is PokerStars. Practice your UTG strategy in full ring, short-handed or even heads-up.
Alternatively read about the pokerstars rakeback, not exactly a rakeback program rather a VIP Club, but potentially very juicy as well.