Your ques has 3 parts:
1. Will Allah forgive ??
2. Can you marry someone else or not ?
3. If you marry another, will u have to tell him all ?
Any sin Allah (swt) will forgive by his ultimate grace, as he is Rahmanur Raheem....more he described about his grace than rage. But condition is REPENTANCE ! REPENTANCE !! and REPENTANCE !!!
There are three conditions to repentance:
a. You should give up the sin
b. You should regret having done it
c. You should resolve never to go back to it
[And Allah knows what you think, what you might think and even what you might not think]
About the sin you committed, Allah says in Quran (interpretation):
"And those who, when they have committed Faahishah (illegal sexual intercourse) or wronged themselves with evil, remember Allaah and ask forgiveness for their sins; — and none can forgive sins but Allaah — and do not persist in what (wrong) they have done, while they know" -- [Al ‘Imraan 3:135]
To see an excellent example of repentance in case of adultery follow the hadiths in the given links:
http://sunnah.com/muslim/29/34
http://sunnah.com/muslim/29/36
To know more read this.
Now answer to your 2nd question, you can not marry someone who is chaste unless you repent. But once you truly repent your sin is waived and you can marry...
To know more read this.
To answer the 3rd part, in any case you should repent to Allah and conceal your sins from anyone else...
Abu Hurayrah said:
I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “All of my ummah will be forgiven except those who sin openly. It is a part of sinning openly when a man does something at night, then the following morning when Allaah has concealed his sin, he says, ‘O So and so, I did such and such last night,’ when all night his Lord has concealed him and the next morning he uncovers what Allaah had concealed.” (Narrated by al-Bukhaari, 5721; Muslim, 2990)...
So you should not confess your sin unless it's open by itself. But if it's open, then the punishment as per Sharia (islamic law) has to be imparted (in your case , as you are unmarried it's 100 lashes of beating)
To know more: islamqa and islamweb