N3
でよければ
If it is alright, If you don't mind, If you would like, If possible
私でよければ手伝うよ。
If I'll do, I can help you.
Explanation
「Nでよければ」 means 'if I (this) will do'. When you offer help, put yourself forward, or offer something that may not be perfect, you say it modestly and leave the choice to the other person: 「私でよければ、手伝いますよ」 (If I'll do, I'm happy to help).
Mixed up with
Why 「で」 and not 「が」: 「私がよければ」 means 'if it suits me', centred on yourself; 「私でよければ」 carries 'I may not be the best choice, but if you don't mind', Japanese modesty.
Useful at work: 「私でよければ、いつでもご相談に乗ります」 (If I can be of help, feel free to talk to me anytime).