trekkie: firstly lucky you.
now you should have a copy of your acceptance letter which you signed what does it say about notice? it should be no more than two weeks since u r a new employee. then (i believe you should have a number by now) pronto call someone in authority you know in the first place you have signed already to inform them verbally of your intention not to work with them anymore and so put them on notice. then ask for time to come in and refuse the appointment formally (in writing) don't allow more than 3 days for this.
on monday you'll resume at your preferred job and then you need to make time during any lunch hour to go over to the first place and process your "resignation" make sure you write the letter before hand and make sure the relevant pple are in the office and expecting you before going over to make the process go faster.

now you may need to eat humble pie at the place you have already signed so just determine that no matter what they say to you, bear it. cos many people wanted that job and they gave it to you. you may not know who will be humiliated by this your refusal to work with them. so grovel if you have to just to be released from your obligation.

leave your dignity at the door so you don't rile someone up 'cos some hr professionals can...just to be bloody minded....choose to make your exit difficult for you.

if you got your job through man-know-man please see the person who connected you and explain yourself hold nothing back and apologize so they'll help you again in the future.

whew! LOL i have written a lot hope you'll read all. and all the best.

trekkie: your signature on the document makes it a legal document even if you have not started working. the principle is that you have agreed to work for them; now i don't expect anyone to go the legal route; at least not in naija anyway.

now why you should not send a text
sending a text is the height of "arifin" which means "insult" and does not speak well of you.
it makes you seem like an ingrate after all you were happy to get the job before the new one came.
i assume that both companies offered you similar positions and you don't really want to start your professional career being perceived as being flippant. you may have reason to interact with this company again. don't burn your bridges.
remember the law of God what goes around comes around.in other words you reap what you sow. do the right thing.
Thanks
you are G̶̲̥̅Ơ̴̴̴̴͡D̶̲̥̅ sent

address the letter to the hr or admin manager or whoever is in charge of your recruitment.
you can just begin by saying RE: APPOINTMENT AS ..........
first thank them for the opportunity to serve in whatever capacity i.e. your job title and because of the regard you have for the company you enthusiastically accepted their offer. (this is to soften the coming blow)
however, for PERSONAL REASONS, you regretfully have to decline the offer.blah blah blah you look forward to working with them again in the future. you are sorry for any inconveniences you may have caused the company (admit nothing on paper hence the words "may have caused" )
thank them once again for the opportunity blah blah blah
yours ....
hope this helps all the best