Resistance Training (weights) can be a very useful path to better strength fitness if you do it right. Firstly, Form (correct posture) is paramount, doing any activity with poor posture is bad for your circulation, makes you more prone to injury, wastes energy and it just looks all wrong. Exercising with weights using poor form magnifies the consequences of bad posture greatly.
Resistance exercise should be performed with a straight back, if the weight cannot be lifted evenly (with right and left hands) it is too heavy for you and if you have to change your posture during your lifting you may injure yourself. Be clear about what you are trying to achieve how you should look when lifting, exercising in front of a mirror may help you.
Even if you think you know what you are doing remain open to criticism and different ideas, it is possible that there are better ways to exercise than what you are accustomed to. A major warning sign with resistance training is sharp and restrictive pain, you may be lucky enough to “work through it” for a while and not injure yourself but not all exercises work for all people.
Always work the agonist and antagonist muscles evenly, this means that it is best for you to have muscles of equilavalent strength opposing each other, eg.- your bicep bends your elbow and your tricep straightens it, exercise both. Acquaint yourself with muscle anatomy it is interesting and easy to understand once you learn what the individual syllables of the words mean.
The best evidence of good strength condition is not huge muscles, it is a symmetrical upright posture, an even upright posture shows a good strength and flexability balance between your agonists and antagonists and the right and left sides of your body. People move with more ease, grace and coordination with good strength balance.
While it can be useful to watch yourself in the mirror to ensure good form be mindful not to fall into the trap of exercising only those muscles you can see in the mirror, on the way to getting big pecs (chest muscles) you may totally neglect the muscles of the upper back and get a very unattractive and unhealthy round shouldered look about you.
If you decide to play it safe and get a personal trainer you will usually achieve better results if you choose one of about your own age because they may be in a better position to know what may be realistically achievable for you. A much younger trainer may be more attractive but will usually have less knowledge and experience than an older one will and not be as aware of age related body changes.
Always check the fasteners before you lift, it is easy (and embarrassing) for a weight slide off the bar and break your foot as you lift.
Allow yourself at least 2 minutes between sets and try to work with your weights at least once in every 3 days. If you have no weights with you rather than miss out use your own body weight for resistance training like push ups and chin ups.