My 100 amp service panel is full. I have no room to add any more circuits, what is needed to do so AS INEXPENSIVE as possible, thanks John :)

I made the mistake of cramming more in with tandem breakers 30 years ago. I am in the process of adding a sub panel. Small ones aren't that expensive. The first step is to read the label of your current box. It should give a maximum number of poles. Each single breaker is one, and the double pole ones are 2. Count what you have. If it is less than the maxim, you can replace existing breakers with tandem ones or half size ones, gaining one circuit for each one you replace. Go read your panel and count poles. If you need more help, post back. Mucking about in the main panel isn't the best place for a DIY, but you will get help here.

In the 30 years I have had an over crowded panel, I haven't had any problems. However, it violates code. If you ever sell, you may be forced to bring the house up to code. May as well do it right now while you have tkrussell to help get it right.

The general rule is the max branch circuit breakers allowed in any panelboard is 42.

Different size, style, and manufacturers panels are limited to the total amount of branch circuit breaker allowed, for various reasons. The amount allowed , as Labman mentions, is listed on the label. This label will also state the model number of breaker allowed.

It may be simple as Labman mentioned, (he has some experience with this) as pulling out one full size breaker and installing one tandem or two half size breakers. All depends on your panel.

Many times the only remedy is to add a subpanel. This is not inexpensive, so may not suit your needs.

Get back with the label info and we can go from there.