First of all, you need to recognize that these hypothetical situations cannot be answered definitively because there is much information that is assumed, or not provided.
Firebug:
If a person gets married, but not in the Church, and then they divorce,
Assumptions:
(1) One or both parties in your scenario are baptized Catholics. Only Catholics are bound by Canon Law to marry in the Catholic form.
(2) They did not apply for or receive a dispensation to marry outside the Catholic form.
**If ** the above assumptions are true then the person has what is called a “lack of form” and their marriage is invalid from the start. If they divorce, they can petition the dioceses to review their marriage and declar it null. They do not go through the full tribunal process, they have a shorter process due to the lack of form.
If the assumptions are not true, and if:
(1) Neither person is Catholic
(2) One are both are Catholic
and they properly applied for a dispensation to marry outside the Catholic form
Then… the marrriage is presumed valid and they would have to go through a full tribunal process to determine validity.
Firebug:
and that person got married in the Church to somebody else, are they commiting any sin?
By virtue of the fact that they married in the Church, you can assume that they dealt with their first marriage through the proper channels and that they were declared free to marry, therefore there is no sin.
The other possible option would be that they committed massive fraud by lieing on all the paperwork that must be filled out and choosing to hide the fact of the first marriage/divorce. In that case, yes, it would be a sin.
Firebug:
Would you consider that the person was married still, to the first spouse?
In the most likely scenario-- a Catholic with a lack of form who subsequently had their marriage declared null and then married in the church-- no they are not married to their first spouse.
In the case of massive fraud, yes they are.
Firebug:
Or since they did not marry in the church, they were never married, and it would be okay, and valid for that person to marry again, but through the Church? Any answers would be much appreciated!
See my above descriptions of possible combinations. You must be more specific as to the actual marriage conditions.