One of the most important concerns for beginners is safety. After a few years, safe practice becomes automatic, but beginners are most in danger of injuring themselves and others.
Some good ideas for safe practice:
Watch your instructor and listen carefully to his or her instructions.
Work with your partners, not against them.
Inform your instructor and your practice partners of any soreness, strain, injury, or illness you may have so that they can take appropriate precautions.
Use common sense. If you have a sore back, sit out techniques or exercises that may aggravate it.
Respect everyone's abilities, both your own and your partner's. Know and repect your limitations and your partner's.
NOTE: Before training with any injury, disability, or illness, get your doctor's professional opinion and follow his or her advice.
Aikido is a life-long endeavour. If you take care of yourself, you may still be practicing when you are 70 years old. You are better served by taking a few months off to let an injury heal than to cut short your training career for the sake of a couple of training sessions.