Don't know what one of the 3 warframes to choose to start?
Excalibur is an offensive high damage output frame. His downside is that he can be squishy in late game play. He also has some crowd control that also opens foes for finishers.
Volt is more of a all around frame. He can do damage, support his team with speed, and defend with shields. His down side is that people find his speed buff annoying if they arnt using melee or even if they are.
Mag is a great support. She can strip enemy armor in a wide range, give overaheilds, and mark targets for destruction. Her down side is that she is also rather squishy in late game and her ability to strip armor isn't great if the enemies arnt dependant on armor.
All of these 3 frames are amazing and are excellent in their own ways. They are all viable and fun as a first choice.