Firstly, its great to hear about your passion for evangelism! Keep up the good work.
Now, about your friend. It can be really tricky to know what to do at times. You are right not to want to force the conversation too much, because your friend might have a good reason for not wanting to talk about it and forcing the conversation might make things worse. My advice, for what it is worth, is that you try to find out why your friend doesn’t seem interested any more. You could ask directly, something like: “Hey, you know that conversation we had last year about Jesus, you seemed really interested at the time, did something happen?” You might try something more indirect as well, like inviting her to something rather than talking to her directly yourself. “You know how we had that conversation last year about Jesus, I was thinking you might like to come along to church.”
Of course, it might simply be that your friend doesn’t want to hear anything more about Jesus right now, and if thats the case then you should respect that for the time being. If you remain friends then there will always be opportunities later, if you push her away because you insist on talking about Christianity with her then you will have no more opportunities.
That being said, here are some lessons that I had to learn the hard way:
1) God is capable of working without your help. Even if your friend won’t talk about Jesus with you doesn’t mean God is helpless! He can be working in ways that you don’t know about inside her heart.
2) God doesn’t rely on you getting it all right all the time! He uses us to bring people into his kingdom, and that is an honour. But he is perfectly capable of doing it without us too. No matter what you do, even if you make the wrong call with your friend and screw up really badly, nothing can put her out of the reach of God.
3) Therefore prayer is the most important evangelistic activity. You have to rely on God… you literally can’t convert someone yourself - conversion is a new creation (2 Cor 5:17).
4) The next most important thing you can do is continue living your christian life publicly with integrity. Your friend knows you are a christian and she will see that in the way you live.
4) Knowing what to do is difficult and different in each situation. Don’t forget to ask God for wisdom, and he will give it to you (James 1:5).
The take home message for evangelism is that you really can’t go too wrong. Be gentle and respectful (1 Pet 3:15-16), bold and courageous (Josh 1:9) and above all trust God to do the work (Zech 4:6). Think, pray, act and speak wisely. It’s hard, but don’t worry. In the end, the battle is God’s (Prov 21:31) and nobody (including you and your friend) can do anything to thwart God’s plan.