The best way to get a wedding speech started is to thank any one and every one important to the bride and groom and every one who helped set up the wedding. Depending on what your role is, you can spend quite a bit of time thanking every one, including the bride and grooms parents for making the wedding possible.
Telling a funny story about either the bride or groom is also a good way to get the speech going smoothly. Make sure the funny story you tell though isn't vulgar or terribly rude and don't go spilling secrets that might hurt the marriage before it even has a chance to get started. This is not the time to air dirty laundry to every one in the family.
Giving the bride and groom some advice that you've learned over the years about relationships, marriage or just general good advice is a great way to end your speech. If you've learnt something over the many years you've lived and you feel like it would benefit the bride and or groom, then by all means, impart your wisdom to them and the entire wedding reception.
If after reading these tips you still are not sure you can write the perfect wedding speech, I give you permission to steal some one else speech and reword it into your own. You will find thousands of websites that offer free wedding speech templates online (including my website) and there are countless books you can buy online and download online right now that give you really good quality speeches and toasts that you can steal and use as your own.
But the main thing to remember is relax! It's all a bit of fun in the end and as long as you don't insult any one or say anything vulgar and rude, every one will think your speech was great.