This is one of those "if you have to ask, you can't do it" questions. There are usually spring fingers connecting the phone to the antenna. The phone ends are soldered, so they maintain a good contact. The antenna end isn't. Remove the back of the phone. Disassemble the phone until you can get to the antenna and the spring fingers connecting it to the motherboard. Clean the ends of the springs with a gauze swab (or something else that's lintless - not a Q-tip) soaked in alcohol. Clean the spots on the antenna where the springs touch the same way. Bend the springs toward the antenna just a bit - maybe 1/32" - to make the connection tighter. DO NOT use anything abrasive, a file, scraping with a knife, etc. The coating is probably no more than 1 mil thick, and the slightest abrasion will take it off, leaving you with a contact that will corrode pretty quickly. Rubbing lightly with gauze is about as abrasive as you should get. Anything that doesn't come off that way (tobacco tar, etc.) means replacing the part. The alcohol has to do the work, not the rubbing.
If, after doing this, you find no improvement, you have a bad wifi radio in the phone.
If, after doing this, your phone stops working - remember that I said you shouldn't be doing it. If the phone is under warranty, let the seller repair or replace it. If it's not, it's probably cheaper to pay to have it fixed.
BTW, a signal of -100dbm or better should be enough. I can still use my wifi, not at too great a speed, from down the block where the signal is around -140dbm.
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