Ok first you need to deside how low you want to go?
Take a ruler and measure down from the wheel arch to the tyre
1) 30mm (3cm)
2) 40mm (4cm)
3) 50mm (5cm)
Now do the same but this time from the front bumpers lowest part ie the lip.
Then look around where you live, if there are loads of speed bumps then you wont want to go Tooooooo low.
Then look to see if your budget allows for springs anwhere between £40-£100 or a full suspension kit with springs and sports shocks anywhere between £150-£300+
Have a word with people in your regional area on here and see if they can recommend good fitters in and around your area.
Mine cost me £300 to have a full suspension kit fitted, lower strut brace, poly bushes all round, Group A anti roll bar and the torsion bars at the back to match the fronts. The car was in for a whole day so I thought that wasnt a bad price.
That may vary depending on where you go tho, and always aprroach the fitter first and say you have some parts you'd like fitting and just tell them what you have and what you would like done
At the end of the day their there to help you out as they want your money, if they seem uninterested or see you as a walking wallet then say ok I'll think about it and move on.
Remember as well that once the items are fitted it will take a few days or longer to settle down and fine its ture ride heigh so if it looks not as low as youd hoped that could change over a few days
Hope thats helps