Usually it's OK to start a melacha - a forbiden work before Shabbos even if the process will be activated during Shabbos, e.g. lighting a candle before Shabbos even though it will burn on Shabbos or setting a trap before Shabbos that will catch an animal on Shabbos.

The exception to this is where the work is noisy, eg leaving on a clothes dryer, TV etc.

One also has to be mindful of benefiting from a fellow Jew's work even when it's not Shabbos for you, eg one shouldn't listen to a radio show from Israel when they are into Shabbos even though you are still some time until Shabbos.

* Timers are OK, e.g. for lights, for a hot plate to go on on Shabbos morning etc.

* It is fine to mail a letter first class on Friday even though you knew it will be delivered on Shabbos day. There are a number of reasons for this, one being that at best you are asking someone who is asking someone else, rather than a direct request to a gentile to do work for you.

* One other exception is leaving on a pot of cholent before shabbos. It's fine to do this, but due to concerns of adjusting a flame we are supposed to cover the flame.

We bring in Shabbos before sunset on Friday. There is a custom to bring it in 18 mins before sunset if possible.

The earliest time one can bring in Shabbos is an hour and a quarter of halachic hours before sunset. If one lit candles before this one would need to light again, beware in summer months when this can be late. For timings of times where you live, check myzmanim.org