Basic grammar question. :) I've decided to lose some weight. - I made a decision in the past and that is the reason I am exercising, dieting, or at least I have a plan to do so now. In short: a past event refers to, describes or explains a present situation. Present perfect. I decided to lose some weight. - I made a decision in the past. It doesn't refer to the present situation. Maybe you've gained weight. Normally we need to know when you decided this. You may hear speakers use "I decided to lose some weight" when they should use "I have decided...", but any good speaker will know how to use present perfect.

I've decided to lose weight/ I decided to lose weight. They are quite similar. So you can say both of them at any situation. If I am wrong, I hope others can correct me :)