Find a light party and get practice being the tank while leve linking. Seriously. Soloing GLA/PLD is the pits.

Try your hardest to keep mob hate off of your fellow party members. It will take every ability the GLA/PLD has at his disposal, especially at your level, but if you get good at it now before 50, it'll become a rhythm to you. In most endgame situations, you don't have time to think about what move you should be doing next. It should just come naturally/instinctively.

I always start off a group with a Provoke, and then flash the other mob (unless you have AoE Flash, which comes later). Then I pop Sentinel and Aegis Boon. With Aegis Boon, it will allow me to immediately do War Drum afterwards which is your best AoE hate move for the time being. By now, unless your party's BLM is being overzealous, you should have hate locked in on you. Pop Rampart after your fellow members have done their weapon skills/combos/casts. Cure anyone who needs it. Cure yourself especially, so that CON/WHMs don't have to proc as much hate trying to keep you healed. Second Wind comes in handy if you're GLA. Use Fast Blade -> Flat Blade on the mob you're on to help keep hate up. You should have Flash, Provoke, Sentinel, and Rampart on cooldown at all times during a fight while tanking.

Really, if you're focused on keeping hate, you're doing your job well. GLA/PLD make only decent DDers unless you're equipped out the wazoo with attack+ STR+ and acc+, which you shouldn't need to be.

Tanking stat-wise, the stats you should prioritize are Emnity + -> Mind -> HP+ -> Vitality in that order (this is arguable, but there's other guides stating why and I find this effective for me).