You'll find holding agro easier once you get Shield Oath. In the mean time, marking a target and using your single target combo on that will hopefully help a lot. There are several ways to mark targets such as macros and such but you can just look under Character>Actions&Traits>Main Commands>Signs. Drag the button to a hot bar and it will pop up a window when you click on it with all the marks on it.

Note, that if your dps are focusing on your mark the main risk to your threat should be the healer. You will need to throw out occasional flashes, particularly if your taking a lot of damage to keep your targets from heading off after your healer. Note however that flash will not break your combo.

Also provoke can be used to help regain threat on a lost target but only if followed up with a high enmity attack. At range, Shield lob has worked well for me for this.

Finally, gear helps a lot. For a tank, Blacksmith, Armourer and Goldsmith have helped me gearing the most. The more damage you cause, the more enmity you generate. The less damage you take the less your healer has to heal you and the less enmity they generate.

As I said at the top though, Shield Oath will make your life a lot easier.